document allowsBackForwardNavigationGestures and convertFilePath

[jcesarmobile]

The wkwebview-engile plugin had an allowsBackForwardNavigationGestures method that has been ported to cordova-ios when the plugin was integrated, but it's undocumented.

Also when the custom scheme was implemented, a convertFilePath method was added to convert file urls to scheme urls. But it's not documented neither.

They are both in window.WkWebView object.

Not sure if this should go in cordova-ios readme or somewhere else in cordova-docs.