CB-7995: document that `FileTransferError.exception` on iOS is never defined.
CB-7995: document that `FileTransferError.exception` on iOS is never defined.

apache/cordova-plugin-file-transfer,bcalvez/cordova-plugin-file-transfer,starquake/cordova-plugin-file-transfer,apache/cordova-plugin-file-transfer,starquake/cordova-plugin-file-transfer,starquake/cordova-plugin-file-transfer,bcalvez/cordova-plugin-file-transfer,starquake/cordova-plugin-file-transfer