doc: contribute: modifying_contributions: clarify DCO requirements

Make clear that modified commits or new commits that incorporate
material from other commits must retain the Signed-off-by: lines from
the original author(s), to preserve the chain-of-responsibility that
affirms the content meets Zephyr's licensing criteria.

Signed-off-by: Peter Bigot <[email protected]>

Author: pabigot

doc/contribute/modifying_contributions.rst

@@ -64,6 +64,13 @@ If the original patches are substantially modified, the developer can either:
   submitted work is based on (mentioning, for example, the original PR
   number).
 
+.. note::
+
+  In any case commits that include content from both the original and
+  modifying author **must** include the ``Signed-off-by`` statements
+  from the original commit that provided the content as well as one from
+  the modifying author.  See :ref:`DCO`.
+
 .. note::
   Contributors should uncheck the box *“Allow Edits By Maintainers"*
   to indicate that they do not wish their patches to be amended,