Update helpdoc.

Author: mneverov

src/cmd/go/internal/help/helpdoc.go

@@ -505,7 +505,7 @@ General-purpose environment variables:
 		The directory where 'go install' will install a command.
 	GOCACHE
 		The absolute path to the directory where the go command will store cached
-		information for reuse in future builds.
+		information for reuse in future builds. Must be an absolute path.
 	GOCACHEPROG
 		A command (with optional space-separated flags) that implements an
 		external go command build cache.