updated comment

Author: andrewseguin

src/lib/sort/sort-header.ts

@@ -293,7 +293,12 @@ export class MatSortHeader implements MatSortable {
   /**
    * Updates the direction the arrow should be pointing. If it is not sorted, the arrow should be
    * facing the start direction. Otherwise if it is sorted, the arrow should point in the currently
-   * active sorted direction.
+   * active sorted direction. The reason this is updated through a function is because the direction
+   * should only be changed at specific times - when deactivated but the hint is displayed and when
+   * the sort is active and the direction changes. Otherwise the arrow's direction should linger
+   * in cases such as the sort becoming deactivated but we want to animate the arrow away while
+   * preserving its direction, even though the next sort direction is actually different and should
+   * only be changed once the arrow displays again (hint or activation).
    */
   _updateArrowDirection() {
     this._arrowDirection = this._isSorted() ?