Documentation on CFG_EXTRAS is misleading

[mathias-luedtke]

The CMake API documentation on CFG_EXTRAS (web version) is misleading as it implies that the template extension should be added to the listed filename.
However only the basename with cmake extension must be listed, as the template extension is detected automatically.
Even worse: If the template extension is given in CFG_EXTRAS the file will be used directly without template expansion!

The documentation should cover this auto-detect feature.
In addition files with extensions should be handled properly or trigger an error.

[dirk-thomas]

Thank you for bringing this up. Can you please propose an update / addition which clarifies this so that it becomes clear to you how the argument works.

[dirk-thomas]

I will close this for now since it is unclear what change would satisfy the request to "clarify" the existing documentation.