show alternate description text for disabled visibility settings

app/assets/stylesheets/pages/settings.scss

@@ -158,15 +158,28 @@
       color: $gl-text-color;
     }
 
-    .option-descr {
+    .option-description,
+    .option-disabled-reason {
       margin-left: 29px;
       color: $project-option-descr-color;
     }
 
+    .option-disabled-reason {
+      display: none;
+    }
+
     &.disabled {
       i.fa {
         opacity: 0.5;
       }
+
+      .option-description {
+        display: none;
+      }
+
+      .option-disabled-reason {
+        display: block;
+      }
     }
   }
 }

app/views/shared/_visibility_radios.html.haml

@@ -8,9 +8,10 @@
       = visibility_level_icon(level)
       .option-title
         = visibility_level_label(level)
-      .option-descr
+      .option-description
         = visibility_level_description(level, form_model)
-- unless restricted_visibility_levels.empty?
-  %div
-    %span.info
-      Some visibility level settings have been restricted by the administrator.
+      .option-disabled-reason
+        - if restricted
+          This visibility level has been restricted by the administrator.
+        - elsif disallowed
+          This option is not available the visibility of parent or child items prevents it.