Documentation: kunit: add description of kunit.enable parameter

Documentation: kunit: add description of kunit.enable parameter

The current KUnit documentation does not mention the kunit.enable
kernel parameter, making it unclear how to troubleshoot cases where
KUnit tests do not run as expected.
Add a note explaining kunit.enable parmaeter. Disabling this parameter
prevents all KUnit tests from running even if CONFIG_KUNIT is enabled.

Link: https://lore.kernel.org/r/20251021030605.41610-1-ishikawa.yuy-00@jp.fujitsu.com
Signed-off-by: Yuya Ishikawa <ishikawa.yuy-00@jp.fujitsu.com>
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>