Document `@editor.completeFrom` and "dot completion everywhere"

[zth]

Some new exciting stuff just landed for the editor tooling: rescript-lang/rescript-vscode#1054

I think we should document these things properly, and potentially other things around the editor tooling as well. Adding this issue as a placeholder to remind to do that.

[fhammerschmidt]

Also document rescript-lang/rescript#7330 when ready