[Buttons] Clarify docs for accessibilityTraitsIncludesButton. (#8328)
[Buttons] Clarify docs for accessibilityTraitsIncludesButton. (#8328)

Adds more detail to the behavior and purpose of
`accessibilityTraitsIncludesButton`. This API is a migration flag and is
intended to allow clients to migrate to the default `UIButton` behavior.

Follow-up to #6763
Reduces ambiguity identified in #8327
material-components/material-components-ios,material-components/material-components-ios,material-components/material-components-ios,material-components/material-components-ios,material-components/material-components-ios