spec: state which predeclared types are defined or alias types (clarification)

When we introduced the distinction between "defined" and "alias" types
we retained the notion of a "named" type (any type with a name). The
predeclared types (which all have names) simply remained named types.

This CL clarifies the spec by stating excplicitly which predeclared
types are defined types (or at least "act" like defined types), and
which ones are alias types.

Fixes #21785.

Change-Id: Ia8ae133509eb5d738e6757b3442c9992355e3535
Reviewed-on: https://go-review.googlesource.com/64591Reviewed-by: Russ Cox <rsc@golang.org>

doc/go_spec.html

 <!--{
 	"Title": "The Go Programming Language Specification",
-	"Subtitle": "Version of September 1, 2017",
+	"Subtitle": "Version of September 19, 2017",
 	"Path": "/ref/spec"
 }-->
 
@@ -761,7 +761,8 @@ using a receiver of that type.
 <p>
 A <i>boolean type</i> represents the set of Boolean truth values
 denoted by the predeclared constants <code>true</code>
-and <code>false</code>. The predeclared boolean type is <code>bool</code>.
+and <code>false</code>. The predeclared boolean type is <code>bool</code>;
+it is a <a href="#Type_definitions">defined type</a>.
 </p>
 
 <h3 id="Numeric_types">Numeric types</h3>
@@ -808,8 +809,9 @@ uintptr  an unsigned integer large enough to store the uninterpreted bits of a p
 </pre>
 
 <p>
-To avoid portability issues all numeric types are distinct except
-<code>byte</code>, which is an alias for <code>uint8</code>, and
+To avoid portability issues all numeric types are <a href="#Type_definitions">defined
+types</a> and thus distinct except
+<code>byte</code>, which is an <a href="#Alias_declarations">alias</a> for <code>uint8</code>, and
 <code>rune</code>, which is an alias for <code>int32</code>.
 Conversions
 are required when different numeric types are mixed in an expression
@@ -825,7 +827,8 @@ A <i>string type</i> represents the set of string values.
 A string value is a (possibly empty) sequence of bytes.
 Strings are immutable: once created,
 it is impossible to change the contents of a string.
-The predeclared string type is <code>string</code>.
+The predeclared string type is <code>string</code>;
+it is a <a href="#Type_definitions">defined type</a>.
 </p>
 
 <p>