posix-timers: Document nanosleep() details

The descriptions for common_nsleep() is wrong and common_nsleep_timens()
lacks any form of comment.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Link: https://lore.kernel.org/r/20230425183313.567072835@linutronix.de