appcache docs and tweak warning message

I've rearranged the documentation: information about the appcache package, features, API, and how to handle large files is now in the docs.  Left in the Wiki page is background information about how the browser uses the app cache and how it interacts with hot code reloads.

The warning now looks like this:

````
** You are using the appcache package but the total size of the
** cached resources is 6.7MB.
**
** This is over the recommended maximum of 5 MB and may break your
** app in some browsers! See http://docs.meteor.com/#appcache
** for more information and fixes.
````