Preparing module CMake documentation for consumption by Doxygen. Updating Doxygen generation to use this will be in a followup (separated out to keep MRs smaller and easier to review).

Note that there are two minor behavioral changes at the root of this topic that should have gone into !6110 (merged) but I had missed them in my rebase sorting.

Cc: @brad.king @demarle @robertmaynard @cobo