This document is ready to be published as an RFC. 

Nits:

General - RFC7807 / RFC 7807 (when used in prose)

Section 3.1.3 - The entire section is bold in the HTML output.

   Current:  The "title" string is advisory, and is included only 
             for users who both are not aware of ...

   Perhaps: The "title" string is advisory and is included only 
            for users who are both unaware of ...

Section 3.2 - a "errors" extension / an "errors" extension

Section 7 - IANA prefers that a registry name not include "Registry" 

Current: "HTTP Problem Types Registry"

Perhaps: "HTTP Problem Types" registry