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>