migration: Write documetation for events capabilites

Reported-by: NJiri Denemark <jdenemar@redhat.com>
Signed-off-by: NJuan Quintela <quintela@redhat.com>

qmp-commands.hx

@@ -3406,6 +3406,7 @@ Enable/Disable migration capabilities
 - "rdma-pin-all": pin all pages when using RDMA during migration
 - "auto-converge": throttle down guest to help convergence of migration
 - "zero-blocks": compress zero blocks during block migration
+- "events": generate events for each migration state change
 
 Arguments: