doc: small clarifications to modules.markdown
doc: small clarifications to modules.markdown

per: https://github.com/joyent/node/pull/8708

A modified version of the original PR submitted by @builtbylane.

Reviewed By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
PR-URL: https://github.com/nodejs/node/pull/2378

dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node