[MINOR][DOC] Minor doc change for YARN credential providers

## What changes were proposed in this pull request?

The configuration `spark.yarn.security.tokens.{service}.enabled` is deprecated. Now we should use `spark.yarn.security.credentials.{service}.enabled`. Some places in the doc is not updated yet.

## How was this patch tested?

N/A. Just doc change.

Please review http://spark.apache.org/contributing.html before opening a pull request.

Author: Liang-Chi Hsieh <viirya@gmail.com>

Closes #16444 from viirya/minor-credential-provider-doc.

docs/running-on-yarn.md

@@ -506,7 +506,7 @@ Spark supports integrating with other security-aware services through Java Servi
 `java.util.ServiceLoader`). To do that, implementations of `org.apache.spark.deploy.yarn.security.ServiceCredentialProvider`
 should be available to Spark by listing their names in the corresponding file in the jar's
 `META-INF/services` directory. These plug-ins can be disabled by setting
-`spark.yarn.security.tokens.{service}.enabled` to `false`, where `{service}` is the name of
+`spark.yarn.security.credentials.{service}.enabled` to `false`, where `{service}` is the name of
 credential provider.
 
 ## Configuring the External Shuffle Service
@@ -570,8 +570,8 @@ the Spark configuration must be set to disable token collection for the services
 The Spark configuration must include the lines:
 
 ```
-spark.yarn.security.tokens.hive.enabled   false
-spark.yarn.security.tokens.hbase.enabled  false
+spark.yarn.security.credentials.hive.enabled   false
+spark.yarn.security.credentials.hbase.enabled  false
 ```
 
 The configuration option `spark.yarn.access.namenodes` must be unset.

resource-managers/yarn/src/main/scala/org/apache/spark/deploy/yarn/security/ConfigurableCredentialManager.scala

@@ -38,6 +38,8 @@ import org.apache.spark.util.Utils
  *
  * Also each credential provider is controlled by
  * spark.yarn.security.credentials.{service}.enabled, it will not be loaded in if set to false.
+ * For example, Hive's credential provider [[HiveCredentialProvider]] can be enabled/disabled by
+ * the configuration spark.yarn.security.credentials.hive.enabled.
  */
 private[yarn] final class ConfigurableCredentialManager(
     sparkConf: SparkConf, hadoopConf: Configuration) extends Logging {