Add @commands, @functions, @options

Add @commands, @functions, @options

For the same reason that @mappings is useful, we want a way to
supplement the auto-generated COMMANDS, FUNCTIONS and OPTIONS sections
with arbitrary additional info.

For now this info will get appended to the section, but in the future I
might look at providing nested @header and @footer annotations to allow
you to force it to one location of the other; eg:

  @commands

  @header

  Stuff that gets prepended to the COMMANDS section.

  @footer

  Stuff that gets appended to the COMMANDS section.