add tag description to the tag index page

this is a terrible hack that only works because sphinx doesn't sanitize
its input for the html builder. we set the title to

  '%s<h4>%s</h4>' % (tag, desc)

so the final html looks like <h1>adventure<h4>...</h4></h1>

this may not play nice with the pdf builder..

docs/sphinx_extensions/dfhack/tool_docs.py

@@ -265,7 +265,7 @@ def init_tag_indices(app):
                 tag, desc = tag_tuple[0], tag_tuple[1]
                 topidx.write(('- `{name} <{name}-tag-index>`\n').format(name=tag))
                 topidx.write(('    {desc}\n').format(desc=desc))
-                register_index(app, tag, '"%s" tag index' % tag)
+                register_index(app, tag, '%s<h4>%s</h4>' % (tag, desc))
 
 
 def register(app):