futex.2: Document FUTEX_LOCK_PI2

futex.2: Document FUTEX_LOCK_PI2

FUTEX_LOCK_PI2 is a new futex operation which was recently introduced into the
Linux kernel. It works exactly like FUTEX_LOCK_PI. However, it has support for
selectable clocks for timeouts. By default CLOCK_MONOTONIC is used. If
FUTEX_CLOCK_REALTIME is specified then the timeout is measured against
CLOCK_REALTIME.

This new operation addresses an inconsistency in the futex interface:
FUTEX_LOCK_PI only works with timeouts based on CLOCK_REALTIME in contrast to
all the other PI operations.

Document the FUTEX_LOCK_PI2 command.

Signed-off-by: Kurt Kanzenbach <kurt@linutronix.de>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Alejandro Colomar <alx.manpages@gmail.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>