kunit: update documentation to describe debugfs representation

Documentation should describe debugfs layout and semantics.
Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Reviewed-by: Frank Rowand <frank.rowand@sony.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

Documentation/dev-tools/kunit/usage.rst

@@ -591,3 +591,17 @@ able to run one test case per invocation.
 
 .. TODO(brendanhiggins@google.com): Add an actual example of an architecture
    dependent KUnit test.
+
+KUnit debugfs representation
+============================
+When kunit test suites are initialized, they create an associated directory
+in /sys/kernel/debug/kunit/<test-suite>.  The directory contains one file
+
+- results: "cat results" displays results of each test case and the results
+  of the entire suite for the last test run.
+
+The debugfs representation is primarily of use when kunit test suites are
+run in a native environment, either as modules or builtin.  Having a way
+to display results like this is valuable as otherwise results can be
+intermixed with other events in dmesg output.  The maximum size of each
+results file is KUNIT_LOG_SIZE bytes (defined in include/kunit/test.h).