Documentation/git-config: fix --get-all description

--get does not fail if a key is multi-valued, it returns the last value
as described in its documentation.  Clarify the description of --get-all
to avoid implying that --get does fail in this case.

Signed-off-by: John Keeping <john@keeping.me.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-config.txt

@@ -86,8 +86,7 @@ OPTIONS
 	found and the last value if multiple key values were found.
 
 --get-all::
-	Like get, but does not fail if the number of values for the key
-	is not exactly one.
+	Like get, but returns all values for a multi-valued key.
 
 --get-regexp::
 	Like --get-all, but interprets the name as a regular expression and