git-daemon(1): assorted improvements.

git-daemon(1): assorted improvements.

Jari Aalto noticed a handful places in git-daemon documentation
that need to be improved.

 * --inetd makes --pid-file to be ignored, in addition to --user
   and --group

 * receive-pack service was not described at all.  We should, if
   only to warn about the security implications of it.

 * There was no example of per repository configuration.

Signed-off-by: Junio C Hamano <gitster@pobox.com>