doc: reference list of language-specific globals

Fixes: https://github.com/nodejs/node/issues/6894
PR-URL: https://github.com/nodejs/node/pull/6900


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Robert Jefe Lindstaedt <robert.lindstaedt@gmail.com>