Documentation/gc: warn against --prune=<now>

"git gc" is safe to run anytime only because it has the built-in
grace period to protect objects that are created by other processes
that are waiting for ref updates to anchor them to the history.  In
order to run with no grace period, the user must make sure that the
repository is quiescent.

Reviewed-by: Matthieu Moy <Matthieu.Moy@grenoble-inp.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-gc.txt

@@ -63,8 +63,11 @@ automatic consolidation of packs.
 --prune=<date>::
 	Prune loose objects older than date (default is 2 weeks ago,
 	overridable by the config variable `gc.pruneExpire`).
-	--prune=all prunes loose objects regardless of their age.
-	--prune is on by default.
+	--prune=all prunes loose objects regardless of their age (do
+	not use --prune=all unless you know exactly what you are doing.
+	Unless the repository is quiescent, you will lose newly created
+	objects that haven't been anchored with the refs and end up
+	corrupting your repository).  --prune is on by default.
 
 --no-prune::
 	Do not prune any loose objects.