Remove use of interval from rtems_task_wake_after() documentation

[Chris Johns]

The Classic API documentation for rtems_task_wake_after() refers to an interval while the call accepts ticks. The exact interval is not known because the period of time to next tick is not known when the call is made.

1. Clarify the wording

2. Recommend new applications use the POSIX based calls nanosleep() and clock_nanosleep() for applications that have intervals in a time base rather than system ticks.

The ​Gemini, Verify the fidelity of RTEMS System Tick issue provides some back ground to this change.

[Joel Sherrill]

This is requested as an outcome of ​RTEMS System Tick Resolution (#30) from NSF NOIRLab / Gemini. This is about verifying the fidelity of RTEMS System Tick. A recent test of our systemTickTest tool inside GemUtils? is discussed here.

​https://gitlab.com/nsf-noirlab/gemini/rtsw/epics-base/epics-base/uploads/3ef07e7417e9e14903de65fd91c8417f/EPICS_Task_Sleep-CCJ.pdf

The intent is to be clear it is "number of ticks" and not an arbitrarily precise "interval". Now that RTEMS can have nanosecond granularity this distinction becomes important.

[Kinsey Moore <kinsey.moore@…>]

In 981ba2d/rtems-docs:

c-user: Update references to rtems_task_wake_after

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

[Kinsey Moore <kinsey.moore@…>]

In 20fc133/rtems-central:

spec/wake-after: Update references to intervals

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 and recommends clock_nanosleep() for
applications requiring sleep for a time-based duration instead of a
count of clock ticks.

Updates #4772

[Sebastian Huber <sebastian.huber@…>]

In be789ee/rtems-docs:

c-user: Update copyright and use glossary terms

Updates #4772.

[Kinsey Moore <kinsey.moore@…>]

In 21429f6/rtems:

rtems: Update references to rtems_task_wake_after

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

[Sebastian Huber <sebastian.huber@…>]

In 63a5c19/rtems-docs:

c-user: Fix typo

Updates #4772.

[Sebastian Huber <sebastian.huber@…>]

In 4935659/rtems:

rtems: Fix typo

Update #4772.