Point to documentation about metadata from doc/sensitive_documents/graphics
It would be good if our documentation about creating documents (especially, graphic ones) pointed to the one (if it exists already) about risks of metadata in such files, and (either directly, or transitively) to the MAT documentation.
Feature Branch: doc/7368-warning_metadata
Parent Task: #8416
Original created by @intrigeri on 7368 (Redmine)