Documentation: add a FILES section for show-ref

A peek at where the refs are kept might help understanding, even if,
as the DESCRIPTION section suggests, direct access is not part of the
public API.

Balance that out with a pointer to update-ref.

Suggested-by: Geoff Russell <geoffrey.russell@gmail.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-show-ref.txt

@@ -163,9 +163,15 @@ flag, so you can do
 
 to get a listing of all tags together with what they dereference.
 
+FILES
+-----
+`.git/refs/*`, `.git/packed-refs`
+
 SEE ALSO
 --------
-linkgit:git-ls-remote[1]
+linkgit:git-ls-remote[1],
+linkgit:git-update-ref[1],
+linkgit:gitrepository-layout[5]
 
 AUTHORS
 -------