go/build: clarify Context.HasSubdir docs

Fixes #17888

Change-Id: I7490b95a03b810a0f7ed1f07f37d7c7b3ac036be
Reviewed-on: https://go-review.googlesource.com/34240
Reviewed-by: Dmitri Shuralyov <shurcool@gmail.com>
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>

src/go/build/build.go

@@ -76,8 +76,9 @@ type Context struct {
 	// If IsDir is nil, Import calls os.Stat and uses the result's IsDir method.
 	IsDir func(path string) bool
 
-	// HasSubdir reports whether dir is a subdirectory of
-	// (perhaps multiple levels below) root.
+	// HasSubdir reports whether dir is lexically a subdirectory of
+	// root, perhaps multiple levels below. It does not try to check
+	// whether dir exists.
 	// If so, HasSubdir sets rel to a slash-separated path that
 	// can be joined to root to produce a path equivalent to dir.
 	// If HasSubdir is nil, Import uses an implementation built on