(#5641) Help text: document that puppet doc takes modulepath, manifestdir, and environment options

Puppet doc didn't have the usual subcommand caveat about configuration settings
being valid command line options, nor did it explicitly call out the three
settings that it actually cares about.

I opted to do the latter, since the number of relevant settings was so small.
Note that --environment is currently broken; this is filed as bug #7907.