Add documentation for snippet area cache settings

cookbook/default-snippets.rst

@@ -35,19 +35,28 @@ and can be assigned to a snippet that uses the respective template.
  </meta>
 
  <areas>
- <area key="menu_social_media_links">
+ <area key="menu_social_media_links" cache-invalidation="true">
  <meta>
  <title lang="en">Menu Social Media Links</title>
  </meta>
  </area>
 
- <area key="footer_social_media_links">
+ <area key="footer_social_media_links" cache-invalidation="false">
  <meta>
  <title lang="en">Footer Social Media Links</title>
  </meta>
  </area>
  </areas>
 
+For each area you can also define if the cache should be invalidated when either the snippet is
+changed or the snippet assigned to the area is modified/removed. This can be done by setting the
+``cache-invalidation`` attribute of the area to ``true`` or ``false``, the default value is ``true``.
+
+.. note::
+
+ Be aware that if a default-snippet is used in a base twig template, modifying the snippet or the
+ area may result in a full cache invalidation of the website.
+
 Loading the default snippet inside of a twig template
 -----------------------------------------------------