kernel/sched: Clean up docs for _pend_thread(), limit scope
kernel/sched: Clean up docs for _pend_thread(), limit scope

The scheduler has a kernel-internal _pend_thread() utility which
sounds like a function which will add an arbitrary thread to a wait_q.
This is essentially unsupportable in SMP, where that thread might
actually be executing on a different CPU.

Thankfully we never used it like that.  The only spots outside the
scheduler that use the API are in pipes and mailbox, which both just
want to pend a DUMMY thread to track the timeout but will never try to
pend a true foreign thread.

Clarify the comment and add an assertion to make sure this promise
isn't broken in the future.

Signed-off-by: Andy Ross <c70f9a6bf6ee0cd69c8af4ce5e6b131945769315@intel.com>

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