Documentation/rtla: Describe exit status

Commit 18682166f6 ("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

Documentation/tools/rtla/common_appendix.rst

@@ -1,3 +1,14 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+EXIT STATUS
+===========
+
+::
+
+ 0  Passed: the test did not hit the stop tracing condition
+ 1  Error: invalid argument
+ 2  Failed: the test hit the stop tracing condition
+
 REPORTING BUGS
 ==============
 Report bugs to <linux-kernel@vger.kernel.org>