HADOOP-14948. Document missing config key hadoop.treat.subject.external. Contributed by Ajay Kumar.

(cherry picked from commit e906108fc98a011630d12a43e557b81d7ef7ea5d)
(cherry picked from commit 1452ba139ea845ad9e82a5902d239b59a789bff3)

Conflicts:
	hadoop-common-project/hadoop-common/src/main/resources/core-default.xml

hadoop-common-project/hadoop-common/src/main/resources/core-default.xml

@@ -2472,5 +2472,18 @@
       in audit logs.
     </description>
   </property>
+  <property>
+    <name>hadoop.treat.subject.external</name>
+    <value>false</value>
+    <description>
+      When creating UGI with UserGroupInformation(Subject), treat the passed
+      subject external if set to true, and assume the owner of the subject
+      should do the credential renewal.
+
+      When true this property will introduce an incompatible change which
+      may require changes in client code. For more details, see the jiras:
+      HADOOP-13805,HADOOP-13558.
+    </description>
+  </property>
 
 </configuration>