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

* 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
- 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.

Start by reviewing the existing Cordova documentation and the related issues #177 and #852. Document the differences between file:// and HTTP(S) origins, their consequences such as postMessage and CORS behavior, and when an embedded SPA needs cordova.js.

Written by the indexing model from the issue text.