diff: Document the --no-dereference option.

Also, --no-dereference should not imply --recurse.

MFC after: 3 days
Sponsored by: Klara, Inc.
Reviewed by: allanjude
Differential Revision: https://reviews.freebsd.org/D45779

(cherry picked from commit 4ce4a0fe14c455c6ec77452bc99cd93ea45737b1)