DDSim: make setupUserFunction be documented in the steering file

[andresailer]

BEGINRELEASENOTES

• DDSim: make setupUserFunction be documented in the steering file
• Fix random typos in comments and doxygen strings
• Geant4Handle: Fix error message when plugin casting did not succeed

ENDRELEASENOTES

[github-actions[bot]]

Test Results

   14 files     14 suites   6h 35m 27s :stopwatch:   363 tests   363 :white_check_mark: 0 :zzz: 0 :x: 2 496 runs  2 496 :white_check_mark: 0 :zzz: 0 :x:

Results for commit 04847f08.