docs(form-field): add aria-labels to form examples (#15130)

Fixes #15024

Author: Suresh918

src/material-examples/form-field-custom-control/example-tel-input-example.html

@@ -1,7 +1,7 @@
 <div [formGroup]="parts" class="example-tel-input-container">
-  <input class="example-tel-input-element" formControlName="area" size="3" (input)="_handleInput()">
+  <input class="example-tel-input-element" formControlName="area" size="3" aria-label="Area code" (input)="_handleInput()">
   <span class="example-tel-input-spacer">&ndash;</span>
-  <input class="example-tel-input-element" formControlName="exchange" size="3" (input)="_handleInput()">
+  <input class="example-tel-input-element" formControlName="exchange" size="3" aria-label="Exchange code" (input)="_handleInput()">
   <span class="example-tel-input-spacer">&ndash;</span>
-  <input class="example-tel-input-element" formControlName="subscriber" size="4" (input)="_handleInput()">
+  <input class="example-tel-input-element" formControlName="subscriber" size="4" aria-label="Subscriber number" (input)="_handleInput()">
 </div>