context information must be sent with a document and accessed through events

When modifying a document, we would like to be able to set a comment that will be sent along the edit event so that it can be set in the document history logs.
For more specific use cases, attaching context information to the document so that it's accessible in events is useful.

This could be performed adding a facet (document adapter) to the document model: facets are kept on the document model all along its event processing.
Context information resetting still needs to be done, maybe do it when event uses this information.