update comment

config/layout.conf

@@ -15,7 +15,7 @@ OVERLAYSDIR="${TEMPDIR}/overlays"
 PACKAGESDIR="${TEMPDIR}/packages"
 
 # This is the path where EzBuild will store entire portage trees, that are necessary for
-# synchronization with the mainline.
+# synchronization with the mainline (usually Gentoo).
 PORTAGESDIR="${TEMPDIR}/portages"
 
 # This specifies the directory where EzBuild will put its temporary files and caches.