Document differences between file-based Cordova app vs HTTP(S) web app

[brodycj]

• While most apps would be based on a http: or https: origin domain, a Cordova app would be based on some kind of a file:// index URI path. I think both this difference and the potential consequences are not clear at all in the Cordova documentation. Examples of potential consequences include:
  • Problems with postMessage call such as https://github.com/apache/cordova/issues/177
  • CORS (??? ???)
• It seems to be not 100% clear that a script tag to include cordova.js is needed when embedding an existing SPA web app in Cordova (see issue #852)
• etc. etc.

[brodycj]

Also related: #943 - improve WebView documentation