Documentation: kunit: add documentation for kunit_tool

Add documentation for the Python script used to build, run, and collect
results from the kernel known as kunit_tool. kunit_tool
(tools/testing/kunit/kunit.py) was already added in previous commits.
Signed-off-by: Brendan Higgins <brendanhiggins@google.com>
Reviewed-by: David Gow <davidgow@google.com>
Cc: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

Documentation/dev-tools/kunit/index.rst

@@ -9,6 +9,7 @@ KUnit - Unit Testing for the Linux Kernel
 
 	start
 	usage
+	kunit-tool
 	api/index
 	faq
 

Documentation/dev-tools/kunit/kunit-tool.rst

+.. SPDX-License-Identifier: GPL-2.0
+
+=================
+kunit_tool How-To
+=================
+
+What is kunit_tool?
+===================
+
+kunit_tool is a script (``tools/testing/kunit/kunit.py``) that aids in building
+the Linux kernel as UML (`User Mode Linux
+<http://user-mode-linux.sourceforge.net/>`_), running KUnit tests, parsing
+the test results and displaying them in a user friendly manner.
+
+What is a kunitconfig?
+======================
+
+It's just a defconfig that kunit_tool looks for in the base directory.
+kunit_tool uses it to generate a .config as you might expect. In addition, it
+verifies that the generated .config contains the CONFIG options in the
+kunitconfig; the reason it does this is so that it is easy to be sure that a
+CONFIG that enables a test actually ends up in the .config.
+
+How do I use kunit_tool?
+========================
+
+If a kunitconfig is present at the root directory, all you have to do is:
+
+.. code-block:: bash
+
+	./tools/testing/kunit/kunit.py run
+
+However, you most likely want to use it with the following options:
+
+.. code-block:: bash
+
+	./tools/testing/kunit/kunit.py run --timeout=30 --jobs=`nproc --all`
+
+- ``--timeout`` sets a maximum amount of time to allow tests to run.
+- ``--jobs`` sets the number of threads to use to build the kernel.
+
+If you just want to use the defconfig that ships with the kernel, you can
+append the ``--defconfig`` flag as well:
+
+.. code-block:: bash
+
+	./tools/testing/kunit/kunit.py run --timeout=30 --jobs=`nproc --all` --defconfig
+
+.. note::
+	This command is particularly helpful for getting started because it
+	just works. No kunitconfig needs to be present.
+
+For a list of all the flags supported by kunit_tool, you can run:
+
+.. code-block:: bash
+
+	./tools/testing/kunit/kunit.py run --help

Documentation/dev-tools/kunit/start.rst

@@ -19,7 +19,10 @@ The wrapper can be run with:
 
 .. code-block:: bash
 
-   ./tools/testing/kunit/kunit.py run
+	./tools/testing/kunit/kunit.py run --defconfig
+
+For more information on this wrapper (also called kunit_tool) checkout the
+:doc:`kunit-tool` page.
 
 Creating a kunitconfig
 ======================