Add xmlrole documentation

Author: i80and

source/reference/eval.txt

@@ -1,6 +1,6 @@
-==============
-Eval Directive
-==============
+=========
+Eval Role
+=========
 
 .. warning:: This extension is both **experimental** and potentially
    slow.

source/reference/xmlrole.txt

@@ -0,0 +1,50 @@
+========
+XML Role
+========
+
+The docutils restructuredText parser does not support nesting roles
+or formatting markup. This makes it impossible to, for example, have a
+monospace-formatted ``ref`` link without making a sphinx extension
+providing each possible permutation of formatting.
+
+The ``xml`` role in the ``xmlrole`` extension allows you to express
+formatting using XML. It accepts the following tags:
+
+.. list-table::
+   :header-rows: 1
+   :widths: 10 25 100
+
+   * - Tag
+     - Attributes
+     - Description
+
+   * - ``em``
+     -
+     - Emphasize the contained text. Typically renders as italics.
+
+   * - ``mono``
+     -
+     - Render the contained text in a monospace font.
+
+   * - ``strong``
+     -
+     - Render the contained text in a bold font.
+
+   * - ``ref``
+     - ``target=<ref>``
+     - Create a link to the docutils reference indicated by ``target``.
+
+   * - ``link``
+     - ``target=<url>``
+     - Create a link to a URL indicated by ``target``.
+
+For example:
+
+.. code-block:: rst
+
+    :xml:`<mono><ref target="binary-bson-dumps"></ref></mono>`
+
+    :xml:`<strong>See: <link target="https://example.com">An Example Link</link></strong>`
+
+
+.. seealso:: `xmlrole.py <https://github.com/mongodb/docs-tools/blob/master/sphinxext/xmlrole.py>`__