docs(material/button): expand a11y section

Expands the buttons accessibility section with additional information on
disabling anchors, icons, and toggle buttons.

(cherry picked from commit 1cc00eb)

Author: jelbourn

src/material/button/button.md

@@ -34,9 +34,22 @@ approach this to the consuming app.
 
 ### Accessibility
 Angular Material uses native `<button>` and `<a>` elements to ensure an accessible experience by
-default. The `<button>` element should be used for any interaction that _performs an action on the
-current page_. The `<a>` element should be used for any interaction that _navigates to another
-view_.
+default. A `<button>` element should be used for any interaction that _performs an action on the
+current page_. An `<a>` element should be used for any interaction that _navigates to another
+URL_. All standard accessibility best practices for buttons and anchors apply to `MatButton`.
 
+#### Disabling anchors
+`MatAnchor` supports disabling an anchor in addition to the features provided by the native
+`<a>` element. When you disable an anchor, the component sets `aria-disabled="true"` and
+`tabindex="-1"`. Always test disabled anchors in your application to ensure compatibility
+with any assistive technology your application supports.
+
+#### Buttons with icons
 Buttons or links containing only icons (such as `mat-fab`, `mat-mini-fab`, and `mat-icon-button`)
-should be given a meaningful label via `aria-label` or `aria-labelledby`.
+should be given a meaningful label via `aria-label` or `aria-labelledby`. [See the documentation
+for `MatIcon`](https://material.angular.io/components/icon) for more
+information on using icons in buttons.
+
+#### Toggle buttons
+[See the documentation for `MatButtonToggle`](https://material.angular.io/components/button-toggle)
+for information on stateful toggle buttons.