Add docs for `queryFallbacks` for *ByRole in dom-testing-library

[iAziz786]

As dom-testing-library now has support for queryFallbacks, I'll add it into the docs so that others will be aware of it. As mentioned by Kent, I'll add this relevant paragraph as part of it and also how to use it.

An element doesn't have multiple roles in a given environment. It has a single one. Multiple roles in the attribute are evaluated from left to right until the environment finds the first role it understands. This is useful when new roles get introduced and you want to start supporting those as well as older environments that don't understand that role (yet).

Please let me know do I have to add anything else?

[afontcu]

Looks like it was closed in #418. Thanks!