Section on pointers, when used as arguments

When making use of bomberstudios' sandbox library files, I had to debug an issue where they were passing an ordinary value in to a method that expected an out-pointer (`[NSURL URLByResolvingBookmarkData:options:relativeToURL:bookmarkDataIsStale:error:]`).

Note that I'm not sure exactly how or whether this was actually supported previously in Sketch, or whether it was just a little bug in the script that's now treated a bit more strictly. So this may be entirely bullsh*t :-)

I did find that the Mocha documentation is hugely useful though, and would highly recommend featuring it prominently in the documentation for Sketch plugin developers.