docs: enhance xenstore.txt with permissions description

The permission scheme of Xenstore nodes is not really covered by
docs/misc/xenstore.txt, other than referring to the Xen wiki.

Add a paragraph explaining the permissions of nodes, and especially
mentioning removal of nodes when a domain has been removed from
Xenstore.

This is part of XSA-419.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Edwin Török <edvin.torok@citrix.com>
Acked-by: Julien Grall <jgrall@amazon.com>

docs/misc/xenstore.txt

@@ -43,6 +43,17 @@ bytes are forbidden; clients specifying relative paths should keep
 them to within 2048 bytes.  (See XENSTORE_*_PATH_MAX in xs_wire.h.)
 
 
+Each node has one or multiple permission entries.  Permissions are
+granted by domain-id, the first permission entry of each node specifies
+the owner of the node.  Permissions of a node can be changed by the
+owner of the node, the owner can only be modified by the control
+domain (usually domain id 0).  The owner always has the right to read
+and write the node, while other permissions can be setup to allow
+read and/or write access.  When a domain is being removed from Xenstore
+nodes owned by that domain will be removed together with all of those
+nodes' children.
+
+
 Communication with xenstore is via either sockets, or event channel
 and shared memory, as specified in io/xs_wire.h: each message in
 either direction is a header formatted as a struct xsd_sockmsg