docs: better max_overflow field label and description for MongoDB bridge

The new label and description for the max_overflow field clarifies the
purpose of the field, which is to manage the maximum number of
additional workers created when all workers in the pool are busy,
allowing more concurrent connections to the MongoDB server during
workload spikes.

Fixes part of:
https://emqx.atlassian.net/browse/EMQX-9714

rel/i18n/emqx_connector_mongo.hocon

@@ -49,10 +49,10 @@ local_threshold.label:
 """Local Threshold"""
 
 max_overflow.desc:
-"""Max Overflow."""
+"""The maximum number of additional workers that can be created when all workers in the pool are busy. This helps to manage temporary spikes in workload by allowing more concurrent connections to the MongoDB server."""
 
 max_overflow.label:
-"""Max Overflow"""
+"""Max Overflow Workers"""
 
 min_heartbeat_period.desc:
 """Controls the minimum amount of time to wait between heartbeats."""