rtems_task_wake_after takes a parameter in terms of a count of clock
ticks and not a measure in a subunit of seconds. This updates
documentation to reflect that. This also makes obvious the caveat about
the first tick wait not being a whole tick and points the user at a
replacement for better accuracy.
Updates #4772
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.
Introduce a background section.
This makes it easier to automatically generate parts of the scheduling
concepts documentation in the future.
Update #3993.
