Update bulk_reassign_agents_request to include batchSize body parameter (#1465)

This change documents the ability to leverage a batchSize body parameter for the bulk_reassign_agents_request which was introduced in the following PR: elastic/kibana#134565

This will help align the documentation with the Kibana API docs: https://www.elastic.co/guide/en/fleet/current/fleet-apis.html#bulkReassignAgents

(cherry picked from commit 421a653)

Author: asmith-elastic

docs/en/ingest-management/fleet/api-generated/rules/fleet-apis-passthru.asciidoc

@@ -7436,6 +7436,7 @@ Any modifications made to this file will be overwritten.
     <div class='model-description'></div>
     <div class="field-items">
       <div class="param">policy_id </div><div class="param-desc"><span class="param-type"><a href="#string">String</a></span> new agent policy id </div>
+      <div class="param">batchSize (optional)</div><div class="param-desc"><span class="param-type"><a href="#number">Big Decimal</a></span>  </div>
 <div class="param">agents </div><div class="param-desc"><span class="param-type"><a href="#bulk_reassign_agents_request_agents">bulk_reassign_agents_request_agents</a></span>  </div>
     </div>  <!-- field-items -->
   </div>