include/hw/boards: add warning about changing deprecation logic

include/hw/boards: add warning about changing deprecation logic

If we change the deprecation logic in include/hw/boards.h, we must make
a corresponding change to docs/conf.py and docs/about/deprecated.rst.
Add comments to these files as a warning to future maintainers to keep
these files in sync.

Tested-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>