timerfd.2: Add documentation for CLOCK_UPTIME and CLOCK_BOOTTIME

Add CLOCK_UPTIME and CLOCK_BOOTTIME options and a quick pointer
to clock_gettime(2) for a better definition of these values.

Reviewed-by: imp (small tweaks and added Xr)
Pull-Request: https://github.com/freebsd/freebsd-src/pull/1302