DISPATCH-1388: Clarify policy restrictions defined by vhost objects

[ChugR]

State more clearly that policy restrictions are applied to client requests
at network ingress only.

As I read the document now it is unclear if a policy restriction defined
by a vhost would be applied to a request originated at a distant point in
the network. Suppose I have two vhosts, vhost1 and vhost2, and two users,
Alice and Bob. Vhost policy is enabled for address "orders":

|"orders" | vhost1 | vhost2 |
+---------+--------+--------+
| Alice | allow | deny |
| Bob | deny | allow |

If Alice creates a receiver for "orders" on vhost1 and Bob creates a
sender for "orders" on vhost2 then the router network will Bob's
sender to send messages to Alice's receiver. This is allowed even though
user Alice is denied access to that address on vhost2 and user Bob
is denied access on vhost1.

There are separate namespaces for users on each vhost. What user Alice
does on vhost1 is unaffected by the namespace restrictions applied to
vhost2. Alice's identity is not propagated to vhost2 for subsequent
authorization checks.