signature: change SignatureNonce length to >= 1 octets

refs #4804

Change-Id: Ia89ad3f5ae3691d0e64cc7f170836178c8c2d0ef
diff --git a/signature.rst b/signature.rst
index b6bd27e..ca89e71 100644
--- a/signature.rst
+++ b/signature.rst
@@ -114,11 +114,10 @@
 
 ::
 
-    SignatureNonce = SIGNATURE-NONCE-TYPE
-                     TLV-LENGTH ; == 4
-                     4OCTET
+    SignatureNonce = SIGNATURE-NONCE-TYPE TLV-LENGTH 1*OCTET
 
 The ``SignatureNonce`` element adds additional assurances that a signature will be unique.
+The recommended minimum length for a ``SignatureNonce`` element is 8 octets.
 
 .. _SignatureTime: