Better documentation on permission checking in ContentProvider.call().

     * interfaces that are cheaper and/or unnatural for a table-like

     * model.

     *

     * <p class="note"><strong>WARNING:</strong> The framework does no permission checking

     * on this entry into the content provider besides the basic ability for the application

     * to get access to the provider at all.  For example, it has no idea whether the call

     * being executed may read or write data in the provider, so can't enforce those

     * individual permissions.  Any implementation of this method <strong>must</strong>

     * do its own permission checks on incoming calls to make sure they are allowed.</p>

     *

     * @param method method name to call.  Opaque to framework, but should not be {@code null}.

     * @param arg provider-defined String argument.  May be {@code null}.

     * @param extras provider-defined Bundle argument.  May be {@code null}.