doc/go1.18: add documentation for changes to go/ast
doc/go1.18: add documentation for changes to go/ast

Updates #47694

Change-Id: Ied26f6345df2c8640d4be5132a05db3897b59009
Reviewed-on: https://go-review.googlesource.com/c/go/+/369096
Trust: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>
Reviewed-by: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>

golang/go,CAFxX/go,nawawi/go,cloudflare/go,golang/go,golang/go,cloudflare/go,golang/go,CAFxX/go,nawawi/go,CAFxX/go,cloudflare/go,golang/go,CAFxX/go,nawawi/go,CAFxX/go,nawawi/go,golang/go,cloudflare/go,golang/go,nawawi/go,CAFxX/go,nawawi/go,CAFxX/go,cloudflare/go,cloudflare/go,nawawi/go,nawawi/go,golang/go,cloudflare/go,cloudflare/go,CAFxX/go