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
"``O`` is the result of some sequence of incoming ``I1,I2,I3`` elements"

The above is not really relevant and it has nothing to do with ordering. The point is if inputs {IA1, IA2, ..., IAn} "cause" outputs {OA1, OA2, ..., OAk} and inputs {IB1, IB2, ..., IBm} "cause" outputs {OB1, OB2, ..., OBl} and all of IAi happened before than all IBi then OAi happens before OBi. 

This definition is not completely precise, the issues here is that n-to-n operations are possible, and the ops can be stateful.