doc/sod.sty: Add indirection for printing modified description names.

doc/sod.sty: Add indirection for printing modified description names.

The previous version just printed `MOD-' in front of the description
category name.  Now:

  * there's a `\descmod/MOD' macro which should decorate an appropriate
    piece of the category name; and

  * the category name macros themselves take an argument which they
    should apply to the part of the name which should be decorated.