[doc][flexible-ipam] Add IPAM behaviors section

Co-authored-by: Lan <luola@vmware.com>
Signed-off-by: gran <gran@vmware.com>

docs/antrea-ipam.md

@@ -218,6 +218,34 @@ metadata:
 A StatefulSet Pod's IP will be kept after Pod restarts, when the IP is allocated from the
 annotated IPPool.
 
+### IPAM behaviors
+
+When `AntreaIPAM` is enabled, `antrea-controller` will watch IPPool CRs and StatefulSets
+from K8s apiserver, and `antrea-agent` will register IPAM driver to CNI server.
+
+#### On IPPool CR create/update event
+
+`antrea-controller` will update IPPool counters, and periodically clean up stale IP addresses.
+
+#### On StatefulSet create event
+
+`antrea-controller` will check annotations and preallocate IPs from the IPPool.
+
+#### On StatefulSet delete event
+
+`antrea-controller` will clean up IP allocations under this StatefulSet.
+
+#### On Pod create
+
+`antrea-agent` will receive a CNI add request, it will check annotations and allocate an IP from
+an IPPool. The IP is a pre-allocated StatefulSet IP, a specified IP or the next available IP in
+the IPPool.
+
+#### On Pod delete
+
+`antrea-agent` will receive a CNI del request and release the IP allocation from the IPPool.
+The pre-allocation status won't be changed.
+
 ### Data path behaviors
 
 When `AntreaIPAM` is enabled, `antrea-agent` will connect the Node's network interface