[PATCH 3/8] Documentation: KUnit: restore note about mrproper in start.rst

[Tales Aparecida]

The "Getting Started" guide should be beginner-friendly, therefore
add a note about the requirement of a clean source tree when running
kunit_tool for the first time, and its related error.

Signed-off-by: Tales Aparecida <tales.aparecida@xxxxxxxxx>
---
 Documentation/dev-tools/kunit/start.rst | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/Documentation/dev-tools/kunit/start.rst b/Documentation/dev-tools/kunit/start.rst
index 165d7964aa13..e4b73adde6d0 100644
--- a/Documentation/dev-tools/kunit/start.rst
+++ b/Documentation/dev-tools/kunit/start.rst
@@ -19,6 +19,22 @@ can run kunit_tool:
 
 	./tools/testing/kunit/kunit.py run
 
+.. note ::
+	You might see the error:
+	"The source tree is not clean, please run 'make ARCH=um mrproper'"
+
+	That happens because internally kunit.py specifies the build directory in
+	the command ``make O=output/dir`` through the argument ``--build_dir``,
+	which is ``.kunit`` by default, and before starting out-of-tree build,
+	the source tree must be clean.
+
+	There's also the same caveats mentioned in the "Build directory for the kernel"
+	section of the :doc:`admin-guide </admin-guide/README>`, that is,
+	after it's used it must be used for all invocations of ``make``.
+	The good news is that it can indeed be solved by running
+	``make ARCH=um mrproper``, just be aware that this will delete the
+	current configuration and all generated files.
+
 If everything worked correctly, you should see the following:
 
 .. code-block::
-- 
2.37.1