dtoc: Update documentation to cover warnings in more detail

When things go wrong it can be confusing to figure out what to change.
Add a few more details to the documentation.

Fix a 'make htmldocs' warning while we are here.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Walter Lozano <walter.lozano@collabora.com>
1 file changed