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 <[e-mail]>
Reviewed-by: Stephen Boyd <[e-mail]>
Signed-off-by: Shuah Khan <[e-mail]>

Documentation/dev-tools/kunit/usage.rst

···
 .. note::
    A test case will only be run if it is associated with a test suite.
 
+``kunit_test_suite(...)`` is a macro which tells the linker to put the specified
+test suite in a special linker section so that it can be run by KUnit either
+after late_init, or when the test module is loaded (depending on whether the
+test was built in or not).
+
 For more information on these types of things see the :doc:`api/test`.
 
 Isolating Behavior