go/types: be more precise in API comment
go/types: be more precise in API comment

Change-Id: I24c4b08091bf3b8734f5dcdb9eac1a3582a4daa8
Reviewed-on: https://go-review.googlesource.com/135116
Reviewed-by: Alan Donovan <7069e9552f9d110b0a0f326b96f22f63fb99dfc1@google.com>

josharian/go,nawawi/go,josharian/go,akutz/go,akutz/go,golang/go,akutz/go,akutz/go,cloudflare/go,CAFxX/go,CAFxX/go,nawawi/go,golang/go,cloudflare/go,CAFxX/go,nawawi/go,nawawi/go,CAFxX/go,cloudflare/go,cloudflare/go,akutz/go,nawawi/go,CAFxX/go,josharian/go,josharian/go,nawawi/go,josharian/go,akutz/go,josharian/go,akutz/go,golang/go,nawawi/go,cloudflare/go,akutz/go,golang/go,cloudflare/go,josharian/go,nawawi/go,golang/go,josharian/go,CAFxX/go,cloudflare/go,CAFxX/go,cloudflare/go,CAFxX/go,golang/go,golang/go,golang/go