ANDROID: GKI: Document GKI KUnit differences

Added details for the new kernel parameter, kunit.enable, and
added a deviation warning at the top-level of the KUnit documentaion.

Bug: 231160619
Signed-off-by: Joe Fradley <joefradley@google.com>
Change-Id: Ia608bda8cd8c4a8a2d7ba84d02ea5c8fd6a6a99c