Documentation/rtla: Mention default priority

Documentation/rtla: Mention default priority

RTLA allows the priority of workload threads to be set using the -P
option. This is covered in docs, but the default state for RTLA's own
user workload (implemented in timerlat_u.c) is not mentioned.

Add mention of the default user workload priority as well as a reference
to osnoise and timerlat tracers for kernel workload priority.

Signed-off-by: Tomas Glozar <tglozar@redhat.com>
Acked-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <20251010083338.478961-7-tglozar@redhat.com>