Users can create ports for IPs outside of allocation_pool

- Indicated that the pool of allocated IPs is set by their cloud provider
- User would actually see the bug when deleting a port, but we don't
document that procedure.
- To close the bug, will also update the Networking API 2.0 doc

Change-Id: I94fafd62a7f9b66dfcf2392186801d6a1e935cd6
Partial-bug: 1078013
Merge "Users can create ports for IPs outside of allocation_pool"