get_fast_modinfo() php docs are incorrect

The behaviour of get_fast_modinfo() has been modified however it's php docs have not been updated. Specifically, the docs say that you can supply "reset" as course id. If you actually do this you get a deprecated warning about this behaviour. The documentation just needs to be updated to explain the two modes of this function (getting course module info and resetting the cache).

I'm not sure why we've got these two bits of functionality in the one function but that's a separate issue.