Add documentation

doc/extdev/nodes.rst

@@ -61,6 +61,7 @@ Special nodes
 .. autoclass:: only
 .. autoclass:: meta
 .. autoclass:: highlightlang
+.. autoclass:: collapsible
 
 You should not need to generate the nodes below in extensions.
 

doc/usage/restructuredtext/directives.rst

@@ -1030,6 +1030,37 @@ Including content based on tags
       This directive is designed to control only content of document.  It could
       not control sections, labels and so on.
 
+.. _collapsible:
+
+Using collapsible content
+-------------------------
+
+.. rst:directive:: .. collapsible:: <summary description>
+
+   For HTML builders, place the content of the directive into a HTML `details
+   disclosure`_ element, with the *summary description* text included as the
+   summary for the element. The *summary description* text is parsed as
+   reStructuredText, and can be broken over multiple lines if required. An
+   example and the equivalent output are shown below:
+
+   .. code-block:: reStructuredText
+
+      .. collapsible:: ``literal`` and **bold** content.
+
+         This is the body of the directive.
+
+   .. collapsible:: ``literal`` and **bold** content.
+
+      This is the body of the directive.
+
+   Only the HTML 5 output format supports collapsible content. For other builders,
+   the body of the directive is rendered in the document and the *summary
+   description* text is ignored.
+
+   .. versionadded:: 5.1
+
+   .. _details disclosure: https://developer.mozilla.org/en-US/docs/Web/HTML/Element/details
+
 .. _table-directives:
 
 Tables