misc fixes to Framebuffer docs

[rolyatmax]

A couple issues

• COLOR_ATTACHMENT_0 misnamed (should be COLOR_ATTACHMENT0) (fixes Framebuffer documentation mistake #1097)
• code sample uses framebuffer.configure({ ... }) instead of framebuffer.update({ ... })
• the framebuffer.initialize documentation erroneously includes texture as a supported alias for COLOR_ATTACHMENT0
• clearBuffers, configure and drawBuffers all look like Framebuffer methods in the docs, but none are implemented. Are those supposed to be there?

[coveralls]

Coverage increased (+5.2%) to 46.45% when pulling 730b3a9 on tb/fix-docs into 0dabf7f on master.