specdomain: Add option to create separate developer and user versions of the documentation

[cschlep]

It would be good to produce separate developer and user versions of the documentation. The user version would only include the public definitions, while the developers could see all the definitions (private, hidden, and public), along with all the variable declarations, etc.

Is there a directive that we can use inside the code documentation to mark individual sections (determined by indentation) as "developer only", and they get ignored in the non-developer version? Something like the already defined .. only:: directive:

.. only:: developer

where the "developer" tag could be specified during the make process?