Add docs for how to use ocall.

Right now there doesn't seem to be any docs on how to use ocall...and I can't seem to figure it out by trial and error myself.

I'm trying to translate:

`(.call (oget % "getDoc") %)` to use `ocall`, with no success. I've tried things such as:

`(ocall % "getDoc")`, `(ocall % "getDoc" %)`, `(ocall % ("getDoc"))`, `(ocall "getDoc" %)`, etc.

I would imagine the first one is what I should be using. But since there's no documentation I can't tell if its breaking because I'm using it wrong, or because of some other error.