cleanup: fix make distclean in doc

Check this one: unlike most of what I've done in cleanup this changes what goes into the distribution and what's in the repository.  If you prefer a different resolution to the inconsistencies I've noted, let me know and I'll rework it.

RRD*.pod were in the repository, but were always generated and were removed by "make distclean", resulting in an unexpected response from "git status".  Resolve this by removing them from the repository.

Also eliminate the wildcards from CLEANFILES, instead explicitly listing all derived files.

Also add a couple missed(?) files to EXTRADIST (PDF documentation and the RRD*.pod files).