So rdoc avoids over-documentation by deciding that some modules & functions (e.g. private methods) are best left ignored and undocumented. Kindly rdoc lets you override some of this helpfulness by passing the '--all' and/or '--promiscuous' at run-time. Great.
However, there are still some modules that are left undocumented for reasons unknown to me, and my google-fu is not up to finding the relevant documentation on documentation tools.
What I am after really is an exhaustive (or nearly exhaustive) list of things that will cause rdoc to ignore parts of code (i.e. #--, :nodoc: etc).