Clarify that signature placeholder is in the ds namespace

Author: kislyuk

signxml/__init__.py

@@ -325,8 +325,9 @@ def sign(self, data, key=None, passphrase=None, cert=None, reference_uri=None, k
             the payload data.
 
         To specify the location of an enveloped signature within **data**, insert a
-        ``<Signature Id="placeholder"></Signature>`` element in **data**. This element will be replaced by the generated
-        signature, and excised when generating the digest.
+        ``<ds:Signature Id="placeholder"></ds:Signature>`` element in **data** (where
+        "ds" is the "http://www.w3.org/2000/09/xmldsig#" namespace). This element will
+        be replaced by the generated signature, and excised when generating the digest.
         """
         if id_attribute is not None:
             self.id_attributes = (id_attribute, )