[patch] Add description of standard_system_servicedirs into man page

[Ray Strode]

Hi,

> Is the base directory spec relevant to the system bus, or would it be OK
> to only search the install location? A system bus looking in ~/.local or
> whatever might be wrong.
Note that the basedir spec has two different environment variables for
system versus user data directories.

Maybe it should just look at $XDG_DATA_DIRS (the system datadir path)
and not $XDG_DATA_HOME  (the user datadir path)?

I guess there may be some advantage to looking in /usr/local/share for
tarball installed things? I don't know that it really matters, though.

--Ray