[doc] Add some documentation around haproxy_vip_binds

Change-Id: I604730f9b71357a1fc298f1d3c50d08fa58eb7dd

doc/source/configure-haproxy.rst

@@ -309,8 +309,28 @@ defined in the ``user_variables.yml`` file:
      - 10.0.0.10
      - 192.168.0.10
 
+Controlling HAProxy front-end binding
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Haproxy frontend can bind either to some specific IP (VIP) address or
+ethernet interface. A variable which controls this behaviour is
+``haproxy_vip_binds``. It is used for the service, unless ``haproxy_bind``
+is defined on the service level. In that case ``service.haproxy_bind``
+has prescedence over ``haproxy_vip_binds``.
+
+``haproxy_vip_binds`` is generated by the role from other "convenience"
+variables, like
+`haproxy_bind_external_lb_vip_address`,
+`haproxy_bind_external_lb_vip_interface`,
+`haproxy_bind_internal_lb_vip_address` and
+`haproxy_bind_internal_lb_vip_interface`.
+
+Though you still can override ``haproxy_vip_binds`` to fine-control
+the binding process of HAProxy instance.
+
+
 Overriding the address haproxy will bind to
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+-------------------------------------------
 
 In some cases you may want to override the default of having haproxy
 bind to the addresses specified in ``external_lb_vip_address`` and
@@ -326,7 +346,7 @@ This can be set in the ``user_variables.yml`` file:
    haproxy_bind_internal_lb_vip_address: 192.168.0.10
 
 Binding haproxy to interface
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+----------------------------
 
 In some cases it might be more convenient to bind haproxy to the interface
 rather then a specific IP address. For example, this is handy if you decide