Clarify doc for git-config --unset-all.

Previous formulation could make it appear as removing all lines
matching a regexp (at least, I was looking for such a flag, and
confused this flag for what I was looking for).

Signed-off-by: Yann Dirson <ydirson@altern.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>

Documentation/git-config.txt

@@ -78,7 +78,7 @@ OPTIONS
 	Remove the line matching the key from config file.
 
 --unset-all::
-	Remove all matching lines from config file.
+	Remove all lines matching the key from config file.
 
 -l, --list::
 	List all variables set in config file.