Docs added.

extensions/source/com/intellij/openapi/extensions/PluginAware.java

@@ -5,8 +5,13 @@
 package com.intellij.openapi.extensions;
 
 /**
+ * Extensions should implement this interface when it is important to find out what particular plugin has provided this extension.
  * @author akireyev
  */
 public interface PluginAware {
+  /**
+   * Called by extensions framework when extension is loaded from plugin.xml descriptor.
+   * @param pluginDescriptor descriptor of the plugin that provided this particular extension.
+   */
   void setPluginDescriptor(PluginDescriptor pluginDescriptor);
 }