docs(form-field): improve a11y of password visibility toggle (#15129)

Fixes #15023

Author: Suresh918

src/material-examples/form-field-prefix-suffix/form-field-prefix-suffix-example.html

@@ -1,7 +1,9 @@
 <div class="example-container">
   <mat-form-field>
     <input matInput placeholder="Enter your password" [type]="hide ? 'password' : 'text'">
-    <mat-icon matSuffix (click)="hide = !hide">{{hide ? 'visibility_off' : 'visibility'}}</mat-icon>
+    <button mat-icon-button matSuffix (click)="hide = !hide" [attr.aria-label]="'Hide password'" [attr.aria-pressed]="hide">
+    <mat-icon>{{hide ? 'visibility_off' : 'visibility'}}</mat-icon>
+    </button>
   </mat-form-field>
 
   <mat-form-field>