doc: document repl on-demand module loading

Fixes: https://github.com/iojs/io.js/issues/992


Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Brendan Ashworth <brendan.ashworth@me.com>
PR-URL: https://github.com/iojs/io.js/pull/1249