Documentation: do not mention .git/refs/* directories

It is an implementation detail that a new tag is created by adding a
file in the .git/refs/tags directory.  The only thing the user needs
to know is that a "git tag" creates a ref in the refs/tags namespace,
and without "-f", it does not overwrite an existing tag.

Inspired by a report from 乙酸鋰 <ch3cooli@gmail.com>; I think I
caught all the existing mention in Documentation/ directory in the
tip of 1.7.9.X maintenance track, but we may have added new ones
since then.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/Documentation/git-describe.txt b/Documentation/git-describe.txt
index 039cce2..72d6bb6 100644
--- a/Documentation/git-describe.txt
+++ b/Documentation/git-describe.txt
@@ -36,12 +36,12 @@
 
 --all::
 	Instead of using only the annotated tags, use any ref
-	found in `.git/refs/`.  This option enables matching
+	found in `refs/` namespace.  This option enables matching
 	any known branch, remote-tracking branch, or lightweight tag.
 
 --tags::
 	Instead of using only the annotated tags, use any tag
-	found in `.git/refs/tags`.  This option enables matching
+	found in `refs/tags` namespace.  This option enables matching
 	a lightweight (non-annotated) tag.
 
 --contains::