go/ast: mention that FieldLists can now be enclosed by brackets

Type parameter lists are stored using ast.FieldLists. Update the
documentation to reflect that the enclosing delimiter may be a bracket.

Change-Id: Id103e7b38975e94a1b521f75695edc10408ad3dd
Reviewed-on: https://go-review.googlesource.com/c/go/+/378014
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/go/ast/ast.go

@@ -224,11 +224,12 @@ func (f *Field) End() token.Pos {
 	return token.NoPos
 }
 
-// A FieldList represents a list of Fields, enclosed by parentheses or braces.
+// A FieldList represents a list of Fields, enclosed by parentheses,
+// curly braces, or square brackets.
 type FieldList struct {
-	Opening token.Pos // position of opening parenthesis/brace, if any
+	Opening token.Pos // position of opening parenthesis/brace/bracket, if any
 	List    []*Field  // field list; or nil
-	Closing token.Pos // position of closing parenthesis/brace, if any
+	Closing token.Pos // position of closing parenthesis/brace/bracket, if any
 }
 
 func (f *FieldList) Pos() token.Pos {