XATTR updates

Our xattr docs, in both SDK and Server, are deliberately sparse, but some improvements can be made when bandwidth is available:

• Only one xattr can be changed by each `mutatein` - 
  "You can only access a single XATTR (excluding virtual ones)
  You can access multiple paths within that XATTR if you wish"
• Many SDKs lack a code snippet - e.g. https://docs.couchbase.com/go-sdk/current/howtos/subdocument-operations.html#extended-attributes