docs: Add description for LB IPAM in operator doc

Operator manages LB IPs when LB IPAM feature is used. Describe it in
operator doc.

Signed-off-by: Yutaro Hayakawa <yutaro.hayakawa@isovalent.com>

Documentation/internals/cilium_operator.rst

@@ -85,6 +85,12 @@ mode:
 
 For more information on IPAM visit :ref:`address_management`.
 
+Load Balancer IP Address Management
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+When :ref:`lb_ipam` is used, Cilium Operator manages IP address
+for ``type: LoadBalancer`` services.
+
 KVStore operations
 ~~~~~~~~~~~~~~~~~~