[texdoc] help msg

[Karl Berry]

    > take account of config files in the help message, though?
    It's not completely straightforward, but it should not be too hard either.

Ok, then I guess it seems to me that the ideal would be to specify where
the default comes from.  E.g., perhaps the help message would include
information like this:

Defaults: -a, -i (built-in); -l (user config).

(Or the information could be given on each option line, of course.  Just
thought it might be cleaner to separate it.)

Another idea would be to have texdoc -f and/or texdoc -v output this
"option state".  Then the help message could just show the built-in
defaults, and say "run -f to see runtime defaults", or words to that
effect.  I kind of like that idea.

Wdyt?

Anyway, the main thing is that it should be easy for the user to
determine what the defaults are should s/he want to know.