Documentation/rtla: Describe exit status

Documentation/rtla: Describe exit status

Commit 18682166f61494072d58 ("rtla: Set distinctive exit value for failed
tests") expands exit status making it useful.

Add section 'EXIT STATUS' and required SPDX-License-Identifier
to the documentation.

Signed-off-by: Costa Shulyupin <costa.shul@redhat.com>
Acked-by: Steven Rostedt (Google) <rostedt@goodmis.org>
[jc: fixed sphinx error caused by missing blank line]
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20250608105531.758809-2-costa.shul@redhat.com