Documentation: kunit: improve example on testing static functions

The documentation on testing static functions using the KUnit macros
VISIBLE_IF_KUNIT and EXPORT_SYMBOL_IF_KUNIT is lacking clarity and
missing key steps in the example. This has caused bugs and confusion
among developers.

Improve wording of description and add missing steps to the example.
This entails adding the "#include <kunit/visibility.h>" line and the
"MODULE_IMPORT_NS(EXPORTED_FOR_KUNIT_TESTING);" line. Both of which were
missing from the original example and key to exposing static functions.

Link: https://lore.kernel.org/r/20250516190631.1214081-1-rmoar@google.com


Signed-off-by: Rae Moar <rmoar@google.com>
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>