doc: call js function in null context

It's good practice now to call JS functions that don't execute in a
specific scope with v8::Null() as the receiver.  Update the addons
documentation.

PR-URL: https://github.com/iojs/io.js/pull/1125


Reviewed-By: Rod Vagg <rod@vagg.org>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>