Adds explaination of `flatMap` semantics

[benlesh]

Basically this is describing RxJS's concatMap. I tried to add it as detailed as possible because "It works like Iterator.prototype.flatMap is only partially true, since there's the element of asynchrony involved here. If all sources and inner sources are synchronous, though, it is exactly like Iterator.prototype.flatMap.

[domfarolino]

A few outstanding comments from a while ago still.. I'll wait for those to get resolved.

[domfarolino]

Since this operator has already made it into the spec https://wicg.github.io/observable/#dom-observable-flatmap, I think this may not be necessary, and seems to have no activity these days anyways. So I'll close it.