config: Complete documentation of --get-regexp

The asciidoc documentation of the --get-regexp option was
incomplete. Add some missing pieces:
 - List the option in SYNOPSIS
 - Mention that key names are printed

Signed-off-by: Frank Lichtenheld <frank@lichtenheld.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-config.txt

@@ -14,6 +14,7 @@ SYNOPSIS
 'git-config' [--system | --global] --replace-all name [value [value_regex]]
 'git-config' [--system | --global] [type] --get name [value_regex]
 'git-config' [--system | --global] [type] --get-all name [value_regex]
+'git-config' [--system | --global] [type] --get-regexp name_regex [value_regex]
 'git-config' [--system | --global] --unset name [value_regex]
 'git-config' [--system | --global] --unset-all name [value_regex]
 'git-config' [--system | --global] --rename-section old_name new_name
@@ -73,6 +74,7 @@ OPTIONS
 
 --get-regexp::
 	Like --get-all, but interprets the name as a regular expression.
+	Also outputs the key names.
 
 --global::
 	For writing options: write to global ~/.gitconfig file rather than