Which cases patch tool can fix ?

[Ben Humphrey]

Is there doc on the patch tool of which cases it fixes ? I recently tried patching 

CatchingUnchecked cases which appear would be straightforward (replace the Exception with RuntimeException). In the past, I've only seen that some cases don't write a patch file but I don't see an error if I attempt to patch a case and no file is created as I would expect. 

I realize the patch tool is experimental. It's a game changer when fixing existing large codebase.