Update documentation for ACTION_CLEAR_APP_CACHE

The actual cache clearing is not async anymore, so we can report the
status to the calling app. Here, we just change the documentation to
make the caller aware of the 3rd possible activity result: EIO.

This value is returned if an error happens while MediaProvider is
clearing the cache. In this case, we don't know how much of the cache
was deleted, if at all.

Test: build
Bug: 155290762
Change-Id: I3a04a9b5b6b07c6ec61e97a4cb0d3bc8b10b0370