Elaborate on the need for using an explicit ID when using multiple flows in documentation

[plc-dev]

Hi, first of, thanks for the great port of the React flow-library!
I'm not sure if a feature request is the appropriate issue type. Beg my pardon if this is not the case.

When implementing multiple flows for my application, I had to explicitly assign separate IDs for the flow-instances and when accessing the internal state through useFlow.
While this makes absolute sense, it took me a bit to figure out the correct setting, as some bugs* occur when not providing separate IDs manually, which led me down the wrong debugging path initially.

Maybe this is worth its own additional example in the documentation of the MultiFlow-example? I'd be willing to provide the additional documentation if you think this would be helpful.

* e.g. "overriding" the mouse-controls and state of the other instance, etc.

[bcakmakoglu]

You don't necessarily need to assign an id, it's a matter of where you call useVueFlow the first time (subject to change in the next major to avoid this sort of confusion).
Read this section of the docs for more about this..