Markup commands documentation: categories to be set

[Nicolas Sceaux]

Hi,

I've just committed a patch regarding builtin markup commands
documentation. It adds categories and property listing.

Someone should set all markup commands categories (I've set most of
them to "other"). Several categories may be attributed to a command,
by using a list of symbols, eg: (music graphic) instead of just a
symbol. In the documentation, a category will be printed by replacing
hyphens by spaces, so fret-diagrams becomes "Fret diagrams".

I hope that I didn't break anything...

nicolas