CORE-20865: update the docs to match functionality (#1725)

Author: LWogan

data/config-schema/src/main/resources/net/corda/schema/configuration/messaging/1.0/corda.messaging.json

@@ -104,7 +104,7 @@
       "default": {}
     },
     "maxAllowedMessageSize": {
-      "description": "The maximum size, in bytes, for messages. Corda breaks messages exceeding this size into smaller messages before sending. This value must be lower than the maximum message size configured on the message bus itself. If the Corda configuration value is set to a value higher than that of the message bus, messages will not be sent.",
+      "description": "The maximum size, in bytes, for messages. Corda breaks messages exceeding this size into smaller messages before sending. This value must be lower than the maximum message size configured on the message bus itself. If the Corda configuration value is set to a value higher than that of the message bus, messages will not be sent. Note: one exception to this rule is that flow start request bodies must not exceed 50% of this value.",
       "type": "integer",
       "default": 972800,
       "minimum": 512000,