Fix #4911 - Do not generate doc for standard RDoc parser generated object

RDoc has some standard parsers for .c,.cc or fortran files (don't ask why).
Unfortunately our html generator doesn't support the data structures
generated by those parsers and we were bombing on unknown methods.
This patch makes sure we generate html only for our own top level objects.

Signed-off-by: Brice Figureau <brice-puppet@daysofwonder.com>