doc: Added a few comments and links to other relevant docs.

Signed-off-by: John Wilkins <john.wilkins@inktank.com>
doc: Minor changes.

Removed arch-related comments.
Added links to relevant docs.
Moved limitations to user management.

Signed-off-by: John Wilkins <john.wilkins@inktank.com>
doc: Added links. Clarified namespace comments. Added limitations.

Signed-off-by: John Wilkins <john.wilkins@inktank.com>