Add documentation for the special symbols when protocol is extended.

[Andrey Antukh]

I'm referring to "function", "object", "number", "string", "array", and "boolean" (instead of js/Function, ...)

More explanation: https://groups.google.com/d/msg/clojurescript/MKEZ9CBU77o/Bm0yxo8yCgAJ

[Andrey Antukh]

More info: https://github.com/cljsinfo/cljs-api-docs/blob/catalog/refs/cljs.core_extend-type.md

[Andrey Antukh]

Done