Updated WebGlRenderer render Method Documentation

The WebGlRenderer.render's forceClear argument forces a
clear -only- for buffers with their .autoClear property is set to true.

Took me a while to find a bug in my code as I didn't understand
these flags overode the forceClear parameter for WebGlRenderer.render
Merge branch 'dev' of https://github.com/gazliddon/three.js into dev