go/importer: update documentation on ForCompiler lookup parameter
go/importer: update documentation on ForCompiler lookup parameter

Change-Id: I80dff181a79858b52ba77c3f38f77a744b423afa
Reviewed-on: https://go-review.googlesource.com/c/go/+/184079
Reviewed-by: Michael Matloob <4562438b12c14e81a47550acde616ff328a909f8@golang.org>

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