Work around an issue with defining class attribute documentation.

See http://stackoverflow.com/questions/9153473/sphinx-values-for-attributes-reported-as-none/39276413

src/ZODB/valuedoc.py

+"""Work around an issue with defining class attribute documentation.
+
+See http://stackoverflow.com/questions/9153473/sphinx-values-for-attributes-reported-as-none/39276413
+"""
+
+class ValueDoc:
+
+    def __init__(self, text):
+        self.text = text
+
+    def __repr__(self):
+        return self.text
+