[networking-guide] Update qos config doc
[networking-guide] Update qos config doc

Add note about how burst value should be configured and what
will be impact if burst value will be set too low or too high.

Change-Id: I42a7b48d7923551d1c4a2e5d51ae7e0230b4ed44

AlekhyaMallina-Vedams/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals