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

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>