Docs should include --define options; ly:set-option explanation

[Mark Polesky]

Dvlprs,

All --define options should be listed in AU 3.2.2.

Currently, "a few interesting options" are listed
with a mention of -dhelp to show the rest. Why not
list them all?

If anyone is worried about cluttering up the page,
the options could be listed on a different page,
accessible by a link from the AU page, much like
"Align" and "Graphic" are linked from the NR
"Formatting Text" page.

Also ly:set-option usage should be explained, 
preferably on the same page, even though it's not 
a command-line option, because it's so related. Or 
maybe a short new section called something like 
"Setting command-line options in lilypond files" 
or "Setting command-line options without the 
command-line" or something like that.

I also happen to know that the command-line 
options are tucked away in scm/lily.scm -- it 
might be nice to mention that somewhere, too.

I began thinking of this reading the NEWS section,
seeing that midi-extension is mentioned there but
its usage isn't explained anywhere in the docs.

What do you guys think?

- Mark