core: documentation: updated, improved, and completed doxygen comments
core: documentation: fixed bitarithm documentation
core: documentation: replaced task w/ thread
core: documentation: fixed typo in license header
documentation: extend description for CONST attribute
bitarithm: remove unused macros
documentation: fixed doxygen for schedstats
Merge pull request #983 from OlegHahm/core_doxygen_update

core: documentation: updated, improved, and completed doxygen comments