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 <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>
Run-TryBot: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>
Reviewed-by: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

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