Document <input type=color alpha colorspace=display-p3>

[annevk]

As per whatwg/html#10456

[Elchi3]

I opened #40260, review appreciated.

[annevk]

Thanks! I skimmed through it and it looks good to me. (Not sure if it's worth pointing out, but the way the default value is returned depends on the attributes set. So you might not always get #000000.)