go/build: document when Context.BuildTags is used.
go/build: document when Context.BuildTags is used.

Context.BuildTags is not set when you read go/build.Default.BuildTags.
It's only used by (*BuildTags).Import, etc.

Fixes: #27320

Change-Id: I97e5f1923c410b48f70be8c15938a7e04a178e3f
Reviewed-on: https://go-review.googlesource.com/c/131975
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

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