Add note on fill-color that its alpha channel doesn't matter when using extrusions

[tmcw]

We have a note here

But need one here

There's precedent, too, since we have a note that fill-color doesn't affect 1px strokes.

[lbud]

Not needed in light of #554. When the new documentation is published this should be clear:
https://github.com/mapbox/mapbox-gl-style-spec/blob/mb-pages/reference/v8.json#L1749

"fill-extrusion-color": {
  "doc": "The base color of the extruded fill. The extrusion's surfaces will be shaded differently based on this color in combination with the root `light` settings. If this color is specified as `rgba` with an alpha component, the alpha component will be ignored; use `fill-extrusion-opacity` to set layer opacity.",

https://github.com/mapbox/mapbox-gl-style-spec/blob/mb-pages/reference/v8.json#L1738

"fill-extrusion-opacity": {
  "doc": "The opacity of the entire fill extrusion layer. This is rendered on a per-layer, not per-feature, basis."

Please reopen if you think they should be clearer; otherwise, docs will be republished with the (tomorrow?) release of gl-js v0.27.0.