Documentation/git-clone: describe --mirror more verbosely

Some people in #linux-rt noticed that describing what "--mirror" option does
with "it mirrors" is way insufficient.

Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Darren 'Some People' Hart <darren@dvhart.com>
Cc: Michael J Gruber <git@drmicha.warpmail.net>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-clone.txt

@@ -128,7 +128,12 @@ objects from the source repository into a pack in the cloned repository.
 	configuration variables are created.
 
 --mirror::
-	Set up a mirror of the remote repository.  This implies `--bare`.
+	Set up a mirror of the source repository.  This implies `--bare`.
+	Compared to `--bare`, `--mirror` not only maps local branches of the
+	source to local branches of the target, it maps all refs (including
+	remote branches, notes etc.) and sets up a refspec configuration such
+	that all these refs are overwritten by a `git remote update` in the
+	target repository.
 
 --origin <name>::
 -o <name>::