[CB- 7057] Improve File API documentation, especially wrt to cordova.file.* properties

As discussed on the google groups, the documentation for the `cordova.file.*` properties is unclear (see post: https://groups.google.com/forum/#!topic/phonegap/W1ZR6WG5XwM). Revised documentation to make it clearer, and also added a table indicating how the properties relate to the file system and the various properties on each path (read-write, private, etc.)

Bug # is CB-7057.