Doxygen documentation of default scheme is missing in parts.

[holke]

Many functions in the t8_default_SHAPE_cxx.hxx files are missing doxygen documentations of input parameters and/or return values.
These should be added.

Since these should be more or less the same for all shapes, they can be edited in bulk.

[Davknapp]

We did a doxygen-cleanup and have an action that checks it in our CI. So it should be fine.