Docs streams flows basics

Starting on docs of "just flow sink and source", the "simple things".
Been going rather slowly though, still very very WIP, don't review in depth yet

refs #16549
Ok, I see what you mean (for a merge like thing (2in, 1out): `A`s come in their defined order, `B`s as well (defined), but `A`s may be interleaved with `B`s (undefined)). I want to highlight that undefined part of it. Zip and Flexi merge are explained in the next paragraph.

I've reworded this section a bit, will push in a few min (adressing other minor comments)