Add documentation for queryFallback

It seems like testing-library#386 hasn't been fixed yet.

Author: nickserv

docs/dom-testing-library/api-queries.md

@@ -605,6 +605,7 @@ getByRole(
  name?: TextMatch,
  normalizer?: NormalizerFn,
  selected?: boolean,
+ queryFallbacks?: boolean,
  }): HTMLElement
 ```
 
@@ -710,6 +711,10 @@ cy.findByRole('dialog').should('exist')
 
 <!--END_DOCUSAURUS_CODE_TABS-->
 
+You may also want to select an element by any role in its fallbacks, rather than the literal value of the `role` attribute. You can use `queryFallbacks: true` to selectively enable this functionality in specific queries.
+
+> 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).
+
 ### `ByTestId`
 
 > getByTestId, queryByTestId, getAllByTestId, queryAllByTestId, findByTestId,