Docs: Clarify behavior for Material blendDst, blendEquation, and blendSrc when null

[gkjohnson]

Also is there a notation for specifying multiple type options in the docs? blendDstAlpha can be either an integer or null, which is not clear when just looking at the type information. Something like this maybe?

[property:Integer|Null blendDstAlpha]

[mrdoob]

Thanks!