epoll_wait.2: Clarify that the timeout is measured against CLOCK_MONOTONIC

The existing page left the choice of clock as ambiguous. My
reading of the kernel implementation is that CLOCK_MONOTONIC is
always used since ep_poll() calls schedule_hrtimeout_range(),
which calls schedule_hrtimeout_range_clock() passing
CLOCK_MONOTONIC.

References:
 http://lxr.free-electrons.com/source/fs/eventpoll.c?v=4.7#L1614
 http://lxr.free-electrons.com/source/kernel/time/hrtimer.c?v=4.7#L1785

The "measured against" terminology was borrowed from nanosleep.2.

I've checked all the way back to Linux v2.6.12 where the
calculation was done using jiffies so I think that this has
effectively always been true.

Signed-off-by: Mike Crowe <mac@mcrowe.com>

man2/epoll_wait.2

@@ -55,6 +55,9 @@ The
 argument specifies the number of milliseconds that
 .BR epoll_wait ()
 will block.
+Time is measured against the
+.B CLOCK_MONOTONIC
+clock.
 The call will block until either:
 .IP * 3
 a file descriptor delivers an event;