Make it more robust.

Doc/tools/sphinxext/pyspecific.py

@@ -58,8 +58,14 @@ class ImplementationDetail(Directive):
     def run(self):
         pnode = nodes.compound(classes=['impl-detail'])
         content = self.content
-        content[0] = '**CPython implementation detail:** ' + content[0]
+        add_text = nodes.strong('CPython implementation detail:',
+                                'CPython implementation detail:')
         self.state.nested_parse(content, self.content_offset, pnode)
+        if pnode.children and isinstance(pnode[0], nodes.paragraph):
+            pnode[0].insert(0, add_text)
+            pnode[0].insert(1, nodes.Text(' '))
+        else:
+            pnode.insert(0, nodes.paragraph('', '', add_text))
         return [pnode]