put a note in empty symbols files

put a note in empty symbols files

We wondered that some symbols files were empty.
The TL;DR is that those are essentially driers (not libs) that registers
themselve on load and are then only driven by callbacks.

To avoid confising others next time looking at it we added a comment to
these symbols files.

Change-Id: I5e8e575d95622ff74875c22ee7c4a98b65a25912
Signed-off-by: Christian Ehrhardt <christian.ehrhardt@canonical.com>