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/131975Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/go/build/build.go

@@ -43,6 +43,7 @@ type Context struct {
 	// Clients creating a new context may customize BuildTags, which
 	// defaults to empty, but it is usually an error to customize ReleaseTags,
 	// which defaults to the list of Go releases the current release is compatible with.
+	// BuildTags is not set for the Default build Context.
 	// In addition to the BuildTags and ReleaseTags, build constraints
 	// consider the values of GOARCH and GOOS as satisfied tags.
 	// The last element in ReleaseTags is assumed to be the current release.