Add docs for plugin isolation

docs/reference/cluster/nodes-info.asciidoc

@@ -40,6 +40,7 @@ plugins per node:
 * `site`: `true` if the plugin is a site plugin
 * `jvm`: `true` if the plugin is a plugin running in the JVM
 * `url`: URL if the plugin is a site plugin
+* `isolation`: whether the plugin is loaded in isolation (`true`) or not (`false`)
 
 The result will look similar to:
 

docs/reference/modules/plugins.asciidoc

@@ -142,6 +142,20 @@ bin/plugin --install mobz/elasticsearch-head --timeout 1m
 bin/plugin --install mobz/elasticsearch-head --timeout 0
 -----------------------------------
 
+added[1.1.0]
+[float]
+==== Plugins isolation
+
+Since Elasticsearch 1.1, by default, each plugin is loaded in _isolation_ (in its dedicated `ClassLoader`) to avoid class clashes between the various plugins and their associated libraries. The default can be changed through the `plugins.isolation` property in `elasticsearch.yml`, by setting it to `false`:
+
+[source,js]
+--------------------------------------------------
+plugins.isolation: false
+--------------------------------------------------
+
+Do note that each plugin can specify its _mandatory_ isolation through the `isolation` property in its `es-plugin.properties` configuration. In this (rare) case, the plugin setting is used, overwriting whatever default used by Elasticsearch.
+
+
 [float]
 [[known-plugins]]
 === Known Plugins