eventKey and key documentation

[RichardLitt]

The documentation around eventKey and key is confusing or missing, as noted in #320. I would like to make a PR to fix this, but I am unclear on a few areas.

Is eventKey only used in react-bootstrap, or is it also standard in react? cc @trentgrover-wf

[trentgrover-wf]

eventKey is a react-bootstrap concept only. key is the standard React concept, but key values are no longer available for use within a component, so we had to switch to using a separate explicit value for some use cases (hence the birth of eventKey)

[RichardLitt]

Ok. Is there a place we can document this, besides here?

[trentgrover-wf]

should probably just be documented alongside the relevant component examples (e.g. something to the effect of DropdownButton has an optional onSelect property...the onSelect function will receive the eventKey value of the selected MenuItem as a parameter...

[mtscout6]

Something to take into account with #419

[RichardLitt]

I'm not sure that that is what is best - eventKey doesn't only interact with onSelect, and it occurs in 21 different examples, not just in DropdownButton. There should be a warning somewhere about what eventKey is - I don't see where it would go on the Component page, but just including it in DropdownButton and then only when dealing with onSelect is limiting in scope.

The Components page is great, but I'm not sure that this sort of thing belongs there - it seems too general. If it does, it should be at the top, somewhere clear to explain what it is. What do you think?

[mtscout6]

Should we decide to go with #431 than this gets easy.

[AlexKVal]

Superceded by #419