Documentation: kunit: Modular tests should not depend on KUNIT=y

When the documentation was updated for modular tests, the dependency on
"KUNIT=y" was forgotten to be updated, now encouraging people to create
tests that cannot be enabled when the KUNIT framework itself is modular.
Fix this by changing the dependency to "KUNIT".

Document when it is appropriate (and required) to depend on "KUNIT=y".

Fixes: c9ef2d3e ("KUnit: Docs: make start.rst example Kconfig follow style.rst")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>