Docs: clarify that AnySubnetRequest is optional
Docs: clarify that AnySubnetRequest is optional

The phrasing of "may not support" was a little confusing, mostly because
I read it as an imperative - "you may not do this" instead of it
being "you can do this if you want"

Change-Id: Ia4fb87a649f1d4bc312f4ad3bb5c2f48a7b6741b

yanheven/neutron,noironetworks/neutron,MaximNevrov/neutron,igor-toga/local-snat,wolverineav/neutron,eayunstack/neutron,noironetworks/neutron,huntxu/neutron,klmitch/neutron,huntxu/neutron,yanheven/neutron,mahak/neutron,bigswitch/neutron,sebrandon1/neutron,chitr/neutron,mahak/neutron,eayunstack/neutron,openstack/neutron,apporc/neutron,jumpojoy/neutron,MaximNevrov/neutron,chitr/neutron,cloudbase/neutron,igor-toga/local-snat,sebrandon1/neutron,dims/neutron,bigswitch/neutron,apporc/neutron,jumpojoy/neutron,wolverineav/neutron,openstack/neutron,cloudbase/neutron,openstack/neutron,dims/neutron,mahak/neutron,klmitch/neutron