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.
Please avoid noise in the commit, if it adds documentation - that's all that it should do.