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 <peff@peff.net>
Helped-by: W. Trevor King <wking@tremily.us>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-daemon.txt

@@ -147,6 +147,13 @@ OPTIONS
 Giving these options is an error when used with `--inetd`; use
 the facility of inet daemon to achieve the same before spawning
 'git daemon' if needed.
++
+Like many programs that switch user id, the daemon does not reset
+environment variables such as `$HOME` when it runs git programs,
+e.g. `upload-pack` and `receive-pack`. When using this option, you
+may also want to set and export `HOME` to point at the home
+directory of `<user>` before starting the daemon, and make sure any
+Git configuration files in that directory are readable by `<user>`.
 
 --enable=<service>::
 --disable=<service>::