lang/ocaml: simplify handling of the documentation distfiles

- Make use of the options helper, DOCS_DISTFILES, and get rid of the
  poorly readable .if ${PORT_OPTIONS:MDOCS} block
- Drop useless `post-extract' target, all needed files are extracted
  automagically by the framework
- GC left-over CMP_LIB_DESC, MODOPT, PATTERN, and DOCSDISTNAME knobs