!str #16484 #16392 Remove String identifier of junctions

* descriptive name can still be defined with OperationAttributes.name
* revisit all factory methods (apply/create) and implement and document
  them in a consistent manner

Refs #16484 
Refs #16392
This is misleading a bit (or at least confusing), perhaps better «Note that a vertex instance describes exactly one place in the `FlowGraph` that multiple flows can be attached to; if you want to have multiple independent merges within the same `FlowGraph` then you will have to create multiple such vertices.»