Documentation/rv: Add docs for the sched monitors

Add man page and kernel documentation for the sched monitors, as sched
is a container of other monitors, document all in the same page.
sched is the first nested monitor, also explain what is a nested monitor
and how enabling containers or children monitors work.

To: Ingo Molnar <mingo@redhat.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: John Kacur <jkacur@redhat.com>
Cc: Clark Williams <williams@redhat.com>
Link: https://lore.kernel.org/20250305140406.350227-9-gmonaco@redhat.com


Signed-off-by: Gabriele Monaco <gmonaco@redhat.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>