Add L7 documentation
Add L7 documentation

This commit adds documentation for the API changes involved in
adding L7 polcy and rule functionality to Octavia. It is the last in
a chain of commits designed to keep the size of each individual commit
manageable / reviewable.

Change-Id: I1f01a2b546dabc8dc8ad3cf512f7be9276243e74
Partially-Implements: blueprint lbaas-l7-rules
Partially-Implements: blueprint layer-7-switching

openstack/octavia,openstack/octavia,openstack/octavia