Add a hint about the fixed_category_positions site setting in the edit category modal.

app/assets/javascripts/discourse/templates/modal/edit_category.js.handlebars

@@ -128,6 +128,8 @@
             <label>{{i18n category.position}}</label>
             {{textField value=position class="position-input"}}
           </section>
+        {{else}}
+          <p>{{i18n category.position_disabled}}</p>
         {{/if}}
       </div>
     {{/unless}}

config/locales/client.en.yml

@@ -1132,6 +1132,7 @@ en:
       this_year: "this year"
       position: "position"
       default_position: "Default Position"
+      position_disabled: 'Categories will be displayed in order of activity. To control the order of categories in lists, enable the "fixed category positions" setting.'
       parent: "Parent Category"
 
     flagging: