builtin: add documentation for builtins

Do this by adding a fake package called builtin. At the moment, godoc will
not present this documentation because the function names are all lower case,
but there are plans to address this.
The print and println functions are undocumented here. I could be talked
into doing them but I'd rather not promote their use.

R=dsymonds, n13m3y3r, r, gri, rsc
CC=golang-dev
https://golang.org/cl/4907041

src/pkg/builtin/builtin.go

+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+/*
+	Package builtin provides documentation for Go's built-in functions.
+	The functions documented here are not actually in package builtin
+	but their descriptions here allow godoc to present documentation
+	for the language's special functions.
+*/
+package builtin
+
+// Type is here for the purposes of documentation only. It is a stand-in
+// for any Go type, but represents the same type for any given function
+// invocation.
+type Type int
+
+// IntegerType is here for the purposes of documentation only. It is a stand-in
+// for any integer type: int, uint, int8 etc.
+type IntegerType int
+
+// FloatType is here for the purposes of documentation only. It is a stand-in
+// for either float type: float32 or float64.
+type FloatType int
+
+// ComplexType is here for the purposes of documentation only. It is a
+// stand-in for either complex type: complex64 or complex128.
+type ComplexType int
+
+// The append built-in function appends elements to the end of a slice. If
+// it has sufficient capacity, the destination is resliced to accommodate the
+// new elements. If it does not, a new underlying array will be allocated.
+// Append returns the updated slice. It is therefore necessary to store the
+// result of append, often in the variable holding the slice itself:
+//	slice = append(slice, elem1, elem2)
+//	slice = append(slice, anotherSlice...)
+func append(slice []Type, elems ...Type) []Type
+
+// The copy built-in function copies elements from a source slice into a
+// destination slice. (As a special case, it also will copy bytes from a
+// string to a slice of bytes.) The source and destination may overlap. Copy
+// returns the number of elements copied, which will be the minimum of
+// len(src) and len(dst).
+func copy(dst, src []Type) int
+
+// The len built-in function returns the length of v, according to its type:
+//	Array: the number of elements in v.
+//	Pointer to array: the number of elements in *v.
+//	Slice, or map: the number of elements in v. If v is nil, len(v) is zero.
+//	String: the number of bytes in v.
+//	Channel: the number of elements queued (unread) in the channel buffer.
+//	If v is nil, len(v) is zero.
+func len(v Type) int
+
+// The cap built-in function returns the capacity of v, according to its type:
+//	Array: the number of elements in v (same as len(v)).
+//	Pointer to array: the number of elements in *v (same as len(v)).
+//	Slice: the maximum length the slice can reach when resliced.
+//	Channel: the maximum channel buffer capacity, in units of elements.
+func cap(v Type) int
+
+// The make built-in function allocates and initializes an object of type
+// slice, map, or chan (only). Like new, the first argument is a type, not a
+// value. Unlike new, make's return type is the same as the type of its
+// argument, not a pointer to it. The specification of the result depends on
+// the type:
+//	Slice: The size specifies the length. The capacity of the slice is
+//	equal to its length. A second integer argument may be provided to
+//	specify a different capacity; it must be no smaller than the
+//	length, so make([]int, 0, 10) allocates a slice of length 0 and
+//	capacity 10.
+//	Map: An initial allocation is made according to the size but the
+//	resulting map has length 0. The size may be omitted, in which case
+//	a small starting size is allocated.
+//	Channel: The channel's buffer is initialized with the specified
+//	buffer capacity. If zero, or the size is omitted, the channel is
+//	unbuffered.
+func make(Type, size IntegerType) Type
+
+// The new built-in function allocates memory. The first argument is a type,
+// not a value, and the value returned is a pointer to a newly
+// allocated zero value of that type.
+func new(Type) *Type
+
+// The complex built-in function constructs a complex value from two
+// floating-point values. The real and imaginary parts must be of the same
+// size, either float32 or float64 (or assignable to them), and the return
+// value will be the corresponding complex type (complex64 for float32,
+// complex128 for float64).
+func complex(r, i FloatType) ComplexType
+
+// The real built-in function returns the real part of the complex number c.
+// The return value will be floating point type corresponding to the type of c.
+func real(c ComplexType) FloatType
+
+// The imaginary built-in function returns the imaginary part of the complex
+// number c. The return value will be floating point type corresponding to
+// the type of c.
+func imag(c ComplexType) FloatType
+
+// The close built-in function closes a channel, which must be either
+// bidirectional or send-only. It should be executed only by the sender,
+// never the receiver, and has the effect of shutting down the channel after
+// the last sent value is received. After the last value has been received
+// from a closed channel c,
+//	x, ok := <-c
+// will set x to the channel element's zero value and ok to false, and select
+// clauses involving c will never execute.
+func close(c chan<- Type)
+
+// The panic built-in function stops normal execution of the current
+// goroutine. When a function F calls panic, normal execution of F stops
+// immediately. Any functions whose execution was deferred by F are run in
+// the usual way, and then F returns to its caller. To the caller G, the
+// invocation of F then behaves like a call to panic, terminating G's
+// execution and running any deferred functions. This continues until all
+// functions in the executing goroutine have stopped, in reverse order. At
+// that point, the program is terminated and the error condition is reported,
+// including the value of the argument to panic. This termination sequence
+// is called panicking and can be controlled by the built-in function
+// recover.
+func panic(v interface{})
+
+// The recover built-in function allows a program to manage behavior of a
+// panicking goroutine. Executing a call to recover inside a deferred
+// function (but not any function called by it) stops the panicking sequence
+// by restoring normal execution and retrieves the error value passed to the
+// call of panic. If recover is called outside the deferred function it will
+// not stop a panicking sequence. In this case, or when the goroutine is not
+// panicking, or if the argument supplied to panic was nil, recover returns
+// nil. Thus the return value from recover reports whether the goroutine is
+// panicking.
+func recover() interface{}