doc: remove unnecessary v8::HandleScopes

Remove unnecessary v8::HandleScope uses from the addons documentation.

C++ API callbacks run in an implicit v8::HandleScope, there is no need
to declare one in the callback function.

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>