magento/graphql-ce#741: Add extension point to set custom parameters to Query Context object

- added extended docs

Author: lenaorobei

app/code/Magento/CustomerGraphQl/etc/graphql/di.xml

@@ -20,4 +20,4 @@
             </argument>
         </arguments>
     </type>
-</config>
+</config>

app/code/Magento/GraphQl/Model/Query/ContextParametersProcessorInterface.php

@@ -8,7 +8,12 @@
 namespace Magento\GraphQl\Model\Query;
 
 /**
- * Extension point for adding custom parameters to context object
+ * Adding custom parameters to context object:
+ *
+ * - Add new contextParametersProcessors item to ContextFactory in the di.xml.
+ * - Class must extend ContextParametersProcessorInterface.
+ * - Implement execute method which adds additional data to the context though extension attributes.
+ * - This data will be present in each resolver.
  */
 interface ContextParametersProcessorInterface
 {