doc: Add "domains" doc to usage guide

Signed-off-by: Stephen Finucane <stephen@that.guru>

doc/usage/restructuredtext/directives.rst

@@ -9,6 +9,8 @@ of explicit markup. While Docutils provides a number of directives, Sphinx
 provides many more and uses directives as one of the primary extension
 mechanisms.
 
+See :doc:`/usage/restructuredtext/domains` for roles added by domains.
+
 .. seealso::
 
    Refer to the :ref:`reStructuredText Primer <rst-directives>` for an overview
@@ -734,7 +736,8 @@ Index-generating markup
 -----------------------
 
 Sphinx automatically creates index entries from all object descriptions (like
-functions, classes or attributes) like discussed in :ref:`domains`.
+functions, classes or attributes) like discussed in
+:doc:`/usage/restructuredtext/domains`.
 
 However, there is also explicit markup available, to make the index more
 comprehensive and enable index entries in documents where information is not

doc/usage/restructuredtext/index.rst

@@ -19,3 +19,4 @@ __ http://docutils.sourceforge.net/rst.html
    roles
    directives
    field-lists
+   domains

doc/usage/restructuredtext/roles.rst

@@ -15,7 +15,7 @@ They are written as ``:rolename:`content```.
    :rst:role:`any` role to find anything or the :rst:role:`py:obj` role to find
    Python objects are very useful for this.
 
-See :ref:`domains` for roles added by domains.
+See :doc:`/usage/restructuredtext/domains` for roles added by domains.
 
 
 .. _xref-syntax: