run lilypond-book on internals

[Graham Percival]

Could we get *all* the internals treated with lilypond-book ?
Currently the docs generated from define-markup-commands.scm are 
processed with lilypond-book, but the docs generated from other
files (like define-grob-properties.scm) go directly to .texi.

It would be nice to add lilypond examples for properties, and we
seem to have sufficient interest to begin this task.

Cheers,
- Graham