MINOR: Add doc for external schemas in JSONConverter (#20429)

This is a follow-up to #19449, which do the following things:

1. Add document to explain `schema.content` only work for sink connector
when  `schemas.enable` set to true.
2. Handle the case that while jsonValue contains the `schema` and
`payload` fields, we should use the corresponding value.

Reviewers: Priyanka K U <[email protected]>, Chia-Ping Tsai
 <[email protected]>

Author: JimmyWang6

connect/json/src/main/java/org/apache/kafka/connect/json/JsonConverter.java

@@ -222,7 +222,7 @@ public Object toConnect(final Schema schema, final JsonNode value) {
     private JsonConverterConfig config;
     private Cache<Schema, ObjectNode> fromConnectSchemaCache;
     private Cache<JsonNode, Schema> toConnectSchemaCache;
-    private Schema schema = null;     // if a schema is provided in config, this schema will be used for all messages
+    private Schema schema = null;     // if a schema is provided in config, this schema will be used for all messages for sink connector
 
     private final JsonSerializer serializer;
     private final JsonDeserializer deserializer;

connect/json/src/main/java/org/apache/kafka/connect/json/JsonConverterConfig.java

@@ -38,7 +38,8 @@ public final class JsonConverterConfig extends ConverterConfig {
 
     public static final String SCHEMA_CONTENT_CONFIG = "schema.content";
     public static final String SCHEMA_CONTENT_DEFAULT = null;
-    private static final String SCHEMA_CONTENT_DOC = "When set, this is used as the schema for all messages. Otherwise, the schema will be included in the content of each message.";
+    private static final String SCHEMA_CONTENT_DOC = "When set, this is used as the schema for all messages, and the schemas within each of the message will be ignored." 
+        + "Otherwise, the schema will be included in the content of each message. This configuration applies only 'schemas.enable' is true, and it exclusively affects the sink connector.";
     private static final String SCHEMA_CONTENT_DISPLAY = "Schema Content";
 
     public static final String SCHEMAS_CACHE_SIZE_CONFIG = "schemas.cache.size";