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>

Author: Daniel Bristot de Oliveira

Documentation/tools/rtla/common_timerlat_options.rst

@@ -26,3 +26,10 @@
         Set the /dev/cpu_dma_latency to *us*, aiming to bound exit from idle latencies.
         *cyclictest* sets this value to *0* by default, use **--dma-latency** *0* to have
         similar results.
+
+**-u**, **--user-threads**
+
+        Set timerlat to run without a workload, and then dispatches user-space workloads
+        to wait on the timerlat_fd. Once the workload is awakes, it goes to sleep again
+        adding so the measurement for the kernel-to-user and user-to-kernel to the tracer
+        output.