doc: clarify that "git daemon --user=<user>" option does not export HOME=~user
doc: clarify that "git daemon --user=<user>" option does not export HOME=~user

The fact that we don't set $HOME may confuse admins who expect
~<user>/.gitconfig to be used, because that is not what we try to
read.  And worse, since 96b9e0e3, a git-daemon started by root is
likely to fail to run at all, as the user we switch to generally
cannot read ~root.

Signed-off-by: Jeff King <696e3fbcf235d40b6ea1ebd61c87cbea79d444e7@peff.net>
Helped-by: W. Trevor King <f75897670c17327c42ab201733fb8a81ff9f4c34@tremily.us>
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