[Docs] Add note about Floating IP QoS

As it might be confusing for users that ID of QoS policy
attached to FIP will be also visible in port's attributes
after associate FIP with port this patch adds small note
about that in QoS documentation.

This patch clarify also rules from which QoS policy will
be instelled by L3 agent in case when different QoS polies
are associated with a port and a floating IP.

Additionally this commit fixes also one typo found in
qos configs document.

Change-Id: I13e49a12be61407fa0f2898bef7deda64a04a1f5
Closes-Bug: 1781892

doc/source/admin/config-qos.rst

@@ -500,7 +500,15 @@ fixed IP ``192.168.222.5``:
 
 .. note::
 
-   For now, the L3 agent floating IP QoS extension only uses
+   The QoS policy attached to a floating IP is not applied to a port,
+   it is applied to an associated floating IP only.
+   Thus the ID of QoS policy attached to a floating IP will not be visible
+   in a port's ``qos_policy_id`` field after asscoating a floating IP to
+   the port. It is only visible in the floating IP attributes.
+
+.. note::
+
+   For now, the L3 agent floating IP QoS extension only supports
    ``bandwidth_limit`` rules. Other rule types (like DSCP marking) will be
    silently ignored for floating IPs. A QoS policy that does not contain any
    ``bandwidth_limit`` rules will have no effect when attached to a
@@ -508,8 +516,8 @@ fixed IP ``192.168.222.5``:
 
    If floating IP is bound to a port, and both have binding QoS bandwidth
    rules, the L3 agent floating IP QoS extension ignores the behavior of
-   the port QoS, and installs the rules on the appropriate device in the
-   router namespace.
+   the port QoS, and installs the rules from the QoS policy associated to the
+   floating IP on the appropriate device in the router namespace.
 
 Each project can have at most one default QoS policy, although it is not
 mandatory. If a default QoS policy is defined, all new networks created within