Added documentation to important module.js methods

**Proposal here:** #7717
**Summary:** Just some basic documentation around the important methods used by require. I also reorganized one or two lines of code that were out of place (ie: a require call happening down the page) or where existing documentation didn't match.

Please let me know if any comments are incorrect / could be improved. This module is a good introduction to Node core (via require curiosity), so good comments that help new developers out would be of (relatively) high value.
`exports` object?