config: Complete documentation of --get-regexp
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 <86a8c2da8527a1c6978bdca6d7986fe14ae147fe@lichtenheld.de>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git