Add blurb about jar distribution (or lack thereof).

docs/monitoring.md

@@ -1156,6 +1156,11 @@ applications. There are two configuration keys available for loading plugins int
 Both take a comma-separated list of class names that implement the
 <code>org.apache.spark.api.plugin.SparkPlugin</code> interface. The two names exist so that it's
 possible for one list to be placed in the Spark default config file, allowing users to
-easily add other plugins from the command line without overwriting the config file's list.
-
-Duplicate plugins are ignored.
+easily add other plugins from the command line without overwriting the config file's list. Duplicate
+plugins are ignored.
+
+Distribution of the jar files containing the plugin code is currently not done by Spark. The user
+or admin should make sure that the jar file is available to Spark applications, for example, by
+including the plugin jar with the Spark distribution. The exception to this rule is the YARN
+backend, where the <code>--jars</code> command line option (or equivalent config entry) can be
+used to make the plugin code available to both executors and cluster-mode drivers.