XATTR documentation missing some important concepts

The current XATTRs documentation does not cover a few important details in the concepts:

1. A discussion of how this relates to the item quota. For all readers of the SDK documentation, it's part of the 20MByte item size.
2. Pointing out that XATTRs cannot be enumerated or otherwise discovered. It's up to the app to know they exist.  The cluster provides no facility for discovering them.
3. A discussion, of virtual xattrs. The TTL was added specifically for requests by the field, so I'm sure other users will need this information.
4. XATTR flags.  These are only publicly exposed through the SDKs, so I believe this is the right place to cover them.  The sdk-rfc has a discussion which can be drawn from.  See https://docs.google.com/document/d/1z3pJCPg77PZ8U8rFAyABuEhHSl68j1n42zP2RyflWZs/edit#heading=h.z5vaan7obomq .