[1.5.x] Documented the limitation on sep argument to Signer

Co-Authored-By: Gavin Wahl <gwahl@fusionbox.com>

Backport of e9a356a from master

docs/topics/signing.txt

@@ -76,8 +76,10 @@ generate signatures. You can use a different secret by passing it to the
 
 .. class:: Signer(key=None, sep=':', salt=None)
 
-    Returns a signer which uses ``key`` to generate signatures and ``sep``
-    to separate values.
+    Returns a signer which uses ``key`` to generate signatures and ``sep`` to
+    separate values. ``sep`` cannot be in the `URL safe base64 alphabet
+    <http://tools.ietf.org/html/rfc4648#section-5>`_.  This alphabet contains
+    alphanumeric characters, hyphens, and underscores.
 
 Using the salt argument
 -----------------------