math/big: document Int.String

Int.String had no documentation and the documentation for Int.Text
did not mention the handling of the nil pointer case.

Change-Id: I9f21921e431c948545b7cabc7829e4b4e574bbe9
Reviewed-on: https://go-review.googlesource.com/c/go/+/175118Reviewed-by: Robert Griesemer <gri@golang.org>

src/math/big/intconv.go

@@ -16,7 +16,8 @@ import (
 // Base must be between 2 and 62, inclusive. The result uses the
 // lower-case letters 'a' to 'z' for digit values 10 to 35, and
 // the upper-case letters 'A' to 'Z' for digit values 36 to 61.
-// No prefix (such as "0x") is added to the string.
+// No prefix (such as "0x") is added to the string. If x is a nil
+// pointer it returns "<nil>".
 func (x *Int) Text(base int) string {
 	if x == nil {
 		return "<nil>"
@@ -33,6 +34,8 @@ func (x *Int) Append(buf []byte, base int) []byte {
 	return append(buf, x.abs.itoa(x.neg, base)...)
 }
 
+// String returns the decimal representation of x as generated by
+// x.Text(10).
 func (x *Int) String() string {
 	return x.Text(10)
 }