HADOOP-17478. Improve the description of hadoop.http.authentication.signature.secret.file (#2628)

Reviewed-by: Wei-Chiu Chuang <weichiu@apache.org>

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

@@ -2847,7 +2847,7 @@
   <value>${user.home}/hadoop-http-auth-signature-secret</value>
   <description>
     The signature secret for signing the authentication tokens.
-    The same secret should be used for RM/NM/NN/DN configurations.
+    A different secret should be used for each service.
   </description>
 </property>