Document the generate options

[balupton]

Also applies to regenerateEveryOptions configuration option.

Options can be found here: https://github.com/bevry/docpad/blob/ccb91da643ebaa2a25524a281e86438fcf9aefd4/src/lib/docpad.coffee#L3281-L3320

Best place to put this is probably the API docs.