Documentation/git-push.txt: explain better cases where --force is dangerous

The behavior of "git push --force" is rather clear when it updates only
one remote ref, but running it when pushing several branches can really
be dangerous. Warn the users a bit more and give them the alternative to
push only one branch.

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

Author: moy

Documentation/git-push.txt

@@ -124,6 +124,15 @@ no `push.default` configuration variable is set.
 	not an ancestor of the local ref used to overwrite it.
 	This flag disables the check.  This can cause the
 	remote repository to lose commits; use it with care.
+	Note that `--force` applies to all the refs that are pushed,
+	hence using it with `push.default` set to `matching` or with
+	multiple push destinations configured with `remote.*.push`
+	may overwrite refs other than the current branch (including
+	local refs that are strictly behind their remote counterpart).
+	To force a push to only one branch, use a `+` in front of the
+	refspec to push	(e.g `git push origin +master` to force a push
+	to the `master`	branch). See the `<refspec>...` section above
+	for details.
 
 --repo=<repository>::
 	This option is only relevant if no <repository> argument is