cmRST: Fix cmake domain directives with newline before argument

The signature directive added by !8243 (merged) will be commonly used with the form:

.. signature::
   some_command(SOME_SIGNATURE)

  Docs for this signature.

Drop the assumption that all CMake domain directives are immediately followed by their argument on the same line.

While at it, replace enum with enum class to improve readability and avoid unintended conversions.