Refactor clean Mmake targets in doc/

doc/Mmakefile:
    Refactor the clean targets in the doc/ directory.  Now it is very clear
    that the 'realclean' and 'distclean' targets do the same thing, and
    'distclean' is only removes a few additional things than 'clean' does.

    This will reduce the chance of problems similar to the one affecting the
    table of contents' in the PDF documents.