Document receiving a binary file from a single intent widget

Since v1.30, we can use the `ex:` intent appearances on fields of binary types. "If the field has a binary type, Collect will use the ClipData contents."

https://github.com/getodk/collect/issues/4214

Similar to the section on binary files at https://docs.getodk.org/launch-apps-from-collect/#external-apps-to-populate-multiple-fields

Start with the binary-files section in the linked external-apps documentation and review issue #4214 for the single-intent behavior. Add documentation explaining how a binary field receives ClipData from one intent widget, and consider the work done when the behavior and relevant usage are clear to readers.

Written by the indexing model from the issue text.