Explain .lock-wscript and move it

This changes the comment to better explain what .lock-wscript is for
(it's for the [obsolete][v0.8.0] node-waf build system), and moves it
next to the other ignore pattern(s) for binary module compilation artifacts.

[v0.8.0]: http://blog.nodejs.org/2012/06/25/node-v0-8-0/