Documentation: kunit: add a brief blurb about kunit_test_suite

Add a brief blurb saying how and when the kunit_test_suite() macro
works to the usage documentation.

Signed-off-by: Brendan Higgins <brendanhiggins@google.com>
Reviewed-by: Stephen Boyd <sboyd@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>