chrt: (man) explain which kernel config options are needed for SCHED_EXT

chrt: (man) explain which kernel config options are needed for SCHED_EXT

On systems running kernel version 6.12+ it is still possible
that the syscall to set SCHED_EXT fails with EINVAL, this can
happen when certain kernel configuration options have not been
set. The list of required options can be found here:
https://docs.kernel.org/scheduler/sched-ext.html#switching-to-and-from-sched-ext

Let's document this in the man page to give users a heads up.

Signed-off-by: Christian Goeschel Ndjomouo <cgoesc2@wgu.edu>