doc: mention existence/purpose of module wrapper

Included a block in the modules.md file to explain the existence and
purpose of the module wrapper.

PR-URL: https://github.com/nodejs/node/pull/6433


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: James M Snell <jasnell@gmail.com>