Feature: Comment on documents

[nichtich]

Some libraries (e.g. ZLB Berlin) allow to comment on titles. This may become part of PAIA or another dedicated API for commenting and tagging.

[nichtich]

This may be about public comments as well but first use case is comments only visible and editable by patron and library staff. This can be done by following additions:

• Add a new optional document field comment with type string
• Add an optional request field doc.comment to PAIA Core methods request, renew and cancel
• Make clear that PAIA servers and clients MAY ignore and modify the content of comment field, depending on local settings and notifications SHOULD be used instead to communicate from library to patron.