Ability to add XML Comments or Processing Instructions to any element

User community requests ability to add XML comments to elements based on schema annotations.

Also processing instructions similarly. This allows ascribing validation or other errors to parts of a specification such as an Inspection/Sanitization Guide for data.

Note that a feature is present to add Processing instructions that contain a dump of what is called the node-cache of computed property values - this is for debugging purposes but does show how to add PI to a node so it illustrates part of what is needed for this new feature.