!fixup fix(material/datepicker): multi-year view changes in datepicker doesn't announce selected year range

Adds comment in html file to connect the filed Firefox issue with the workaround.

Fixes #28360

Author: essjay05

src/material/datepicker/calendar-header.html

@@ -1,5 +1,8 @@
 <div class="mat-calendar-header">
   <div class="mat-calendar-controls">
+    <!-- [Firefox Issue: https://bugzilla.mozilla.org/show_bug.cgi?id=1880533] 
+      Relocated label next to related button and made visually hidden via cdk-visually-hidden
+      to enable label to appear in a11y tree for SR when using Firefox -->
     <label [id]="_periodButtonLabelId" class="cdk-visually-hidden">{{periodButtonDescription}}</label>
     <button mat-button type="button" class="mat-calendar-period-button"
             (click)="currentPeriodClicked()" [attr.aria-label]="periodButtonLabel"