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

kernel/time/posix-timers.c

@@ -1370,7 +1370,7 @@ SYSCALL_DEFINE2(clock_getres_time32, clockid_t, which_clock,
 #endif
 
 /*
- * nanosleep for monotonic and realtime clocks
+ * sys_clock_nanosleep() for CLOCK_REALTIME and CLOCK_TAI
  */
 static int common_nsleep(const clockid_t which_clock, int flags,
 			 const struct timespec64 *rqtp)
@@ -1382,6 +1382,11 @@ static int common_nsleep(const clockid_t which_clock, int flags,
 				 which_clock);
 }
 
+/*
+ * sys_clock_nanosleep() for CLOCK_MONOTONIC and CLOCK_BOOTTIME
+ *
+ * Absolute nanosleeps for these clocks are time-namespace adjusted.
+ */
 static int common_nsleep_timens(const clockid_t which_clock, int flags,
 				const struct timespec64 *rqtp)
 {