[SPARK-27687][SS] Rename Kafka consumer cache capacity conf and document caching

## What changes were proposed in this pull request?

Kafka related Spark parameters has to start with `spark.kafka.` and not with `spark.sql.`. Because of this I've renamed `spark.sql.kafkaConsumerCache.capacity`.

Since Kafka consumer caching is not documented I've added this also.

## How was this patch tested?

Existing + added unit test.

```
cd docs
SKIP_API=1 jekyll build
```
and manual webpage check.

Closes #24590 from gaborgsomogyi/SPARK-27687.

Authored-by: Gabor Somogyi <gabor.g.somogyi@gmail.com>
Signed-off-by: Dongjoon Hyun <dhyun@apple.com>

core/src/main/scala/org/apache/spark/SparkConf.scala

@@ -714,7 +714,9 @@ private[spark] object SparkConf extends Logging {
       AlternateConfig("spark.yarn.kerberos.relogin.period", "3.0")),
     KERBEROS_FILESYSTEMS_TO_ACCESS.key -> Seq(
       AlternateConfig("spark.yarn.access.namenodes", "2.2"),
-      AlternateConfig("spark.yarn.access.hadoopFileSystems", "3.0"))
+      AlternateConfig("spark.yarn.access.hadoopFileSystems", "3.0")),
+    "spark.kafka.consumer.cache.capacity" -> Seq(
+      AlternateConfig("spark.sql.kafkaConsumerCache.capacity", "3.0"))
   )
 
   /**

docs/structured-streaming-kafka-integration.md

@@ -416,6 +416,24 @@ The following configurations are optional:
 </tr>
 </table>
 
+### Consumer Caching
+
+It's time-consuming to initialize Kafka consumers, especially in streaming scenarios where processing time is a key factor.
+Because of this, Spark caches Kafka consumers on executors. The caching key is built up from the following information:
+* Topic name
+* Topic partition
+* Group ID
+
+The size of the cache is limited by <code>spark.kafka.consumer.cache.capacity</code> (default: 64).
+If this threshold is reached, it tries to remove the least-used entry that is currently not in use.
+If it cannot be removed, then the cache will keep growing. In the worst case, the cache will grow to
+the max number of concurrent tasks that can run in the executor (that is, number of tasks slots),
+after which it will never reduce.
+
+If a task fails for any reason the new task is executed with a newly created Kafka consumer for safety reasons.
+At the same time the cached Kafka consumer which was used in the failed execution will be invalidated. Here it has to
+be emphasized it will not be closed if any other task is using it.
+
 ## Writing Data to Kafka
 
 Here, we describe the support for writing Streaming Queries and Batch Queries to Apache Kafka. Take note that

external/kafka-0-10-sql/src/main/scala/org/apache/spark/sql/kafka010/package.scala

@@ -33,8 +33,9 @@ package object kafka010 {   // scalastyle:ignore
       .createWithDefaultString("10m")
 
   private[kafka010] val CONSUMER_CACHE_CAPACITY =
-    ConfigBuilder("spark.sql.kafkaConsumerCache.capacity")
-      .doc("The size of consumers cached.")
+    ConfigBuilder("spark.kafka.consumer.cache.capacity")
+      .doc("The maximum number of consumers cached. Please note it's a soft limit" +
+        " (check Structured Streaming Kafka integration guide for further details).")
       .intConf
       .createWithDefault(64)
 }

external/kafka-0-10-sql/src/test/scala/org/apache/spark/sql/kafka010/KafkaSparkConfSuite.scala

@@ -0,0 +1,30 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.spark.sql.kafka010
+
+import org.apache.spark.{LocalSparkContext, SparkConf, SparkFunSuite}
+import org.apache.spark.util.ResetSystemProperties
+
+class KafkaSparkConfSuite extends SparkFunSuite with LocalSparkContext with ResetSystemProperties {
+  test("deprecated configs") {
+    val conf = new SparkConf()
+
+    conf.set("spark.sql.kafkaConsumerCache.capacity", "32")
+    assert(conf.get(CONSUMER_CACHE_CAPACITY) === 32)
+  }
+}