Documentation: remove overlay-notes reference to non-existent file

File dt-object-internal.txt does not exist.  Remove a reference to it
and fix up tags for references to other files.

Reported-by:  <afaerber@suse.de>
Signed-off-by: Frank Rowand <frank.rowand@sony.com>
Signed-off-by: Rob Herring <robh@kernel.org>