message: Add comment explaining z-index
diff: @@ -88,6 +88,6 @@ limitations under the License.
  .mx_CompoundDialog_footer {
  box-shadow: 0px -4px 4px rgba(0, 0, 0, 0.05); /* hardcoded colour for both themes */
  - z-index: 1;
  + z-index: 1; /* needed to make footer & shadow appear above dialog content */
  }
  }
  