go/types: minor simplification in assignableTo (cleanup)
go/types: minor simplification in assignableTo (cleanup)

Also, clarify doc string.

Change-Id: If1c5f8e29e2c2080dd899ef76196e97b7b992389
Reviewed-on: https://go-review.googlesource.com/c/go/+/302758
Trust: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>
Reviewed-by: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>

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