Clarify that action can update an entity

See this `rabbitmq-users` discussion:

https://groups.google.com/d/topic/rabbitmq-users/8H50ivEe8nY/discussion

deps/rabbitmq_management/priv/www/js/tmpl/limits.ejs

@@ -83,7 +83,7 @@
                     </td>
                 </tr>
             </table>
-            <input type="submit" value="Add limit"/>
+            <input type="submit" value="Set / update limit"/>
         </form>
     </div>
 </div>

deps/rabbitmq_management/priv/www/js/tmpl/policies.ejs

@@ -151,7 +151,7 @@
           <td class="t"><span class="mand">*</span></td>
         </tr>
       </table>
-      <input type="submit" value="Add policy"/>
+      <input type="submit" value="Add / update policy"/>
     </form>
   </div>
 </div>
@@ -288,7 +288,7 @@
           <td class="t"><span class="mand">*</span></td>
         </tr>
       </table>
-      <input type="submit" value="Add policy"/>
+      <input type="submit" value="Add / update operator policy"/>
     </form>
   </div>
 </div>

deps/rabbitmq_management/priv/www/js/tmpl/users.ejs

@@ -38,7 +38,7 @@
 </div>
 
 <div class="section-hidden">
-  <h2>Add a user</h2>
+  <h2>Add / update a user</h2>
   <div class="hider">
     <form action="#/users-add" method="put">
       <table class="form">
@@ -92,7 +92,7 @@
           </td>
         </tr>
       </table>
-      <input type="submit" value="Add user"/>
+      <input type="submit" value="Add / update user"/>
     </form>
   </div>
 </div>