Mention that resetting the processor usage time of tasks has no impact
on the period status and statistics.
Remove no longer relevant RTEMS_NOT_DEFINED error status.
Close#4528.
The documentation is a consolidation of the comments in Doxygen markup
and the documentation sources in Sphinx markup. The documentation was
transfered to interface specification items. The documentation source
files were generated from the items by a script.
Update #3993.
