virtual xattrs aren't described sufficiently

The meaning and interface stability of XATTRs isn't covered in the documentation. The work under DOC-2663 adds something (it's not even example code, see https://developer.couchbase.com/documentation/server/5.0/developer-guide/extended-attributes-fundamentals.html) but doesn't describe the virtual xattrs, what versions they're available in, and how they can be used.

This came up recently when a user asked for information on creation time/modification time. There is no reference to point the user to.