Documentation: Add tools/rtla timerlat -u option documentation

Add the -u/--user-thread option documentation for timerlat top/hist.

Link: https://lkml.kernel.org/r/bf727c8ccb8f50792200ae620141e047edf4af7a.1686066600.git.bristot@kernel.org



Cc: William White <chwhite@redhat.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Tested-by: Juri Lelli <juri.lelli@redhat.com>
Signed-off-by: Daniel Bristot de Oliveira <bristot@kernel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>