Add docs for is-fullwidth modifier for buttons

docs/documentation/elements/button.html

@@ -49,6 +49,13 @@ meta:
 <a class="button is-large">Large</a>
 {% endcapture %}
 
+{% capture button_displays_example %}
+<a class="button is-small is-fullwidth">Small</a>
+<a class="button is-fullwidth">Normal</a>
+<a class="button is-medium is-fullwidth">Medium</a>
+<a class="button is-large is-fullwidth">Large</a>
+{% endcapture %}
+
 {% capture button_outlined_example %}
 <a class="button is-outlined">Outlined</a>
 <a class="button is-primary is-outlined">Outlined</a>
@@ -497,6 +504,10 @@ meta:
 
 {% include elements/snippet.html wrapper="buttons" content=button_sizes_example %}
 
+{% include elements/anchor.html name="Displays" %}
+
+{% include elements/snippet.html wrapper="buttons" content=button_displays_example %}
+
 {% include elements/anchor.html name="Styles" %}
 
 <h4 class="subtitle">Outlined</h4>