Documentation: KUnit: Update the instructions on how to test static functions

Now that we have the VISIBLE_IF_KUNIT and EXPORT_SYMBOL_IF_KUNIT macros,
update the instructions to recommend this way of testing static
functions.

Signed-off-by: Arthur Grillo <arthurgrillo@riseup.net>
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>