specify in the man page that {cc} is a 2-letter ISA3166 code

doc/tor.1.txt

@@ -787,7 +787,8 @@ The following options are useful only for clients (that is, if
 
 [[ExcludeNodes]] **ExcludeNodes** __node__,__node__,__...__::
     A list of identity fingerprints, country codes, and address
-    patterns of nodes to avoid when building a circuit. Country codes must
+    patterns of nodes to avoid when building a circuit. Country codes are
+    2-letter ISA3166 codes, and must
     be wrapped in braces; fingerprints may be preceded by a dollar sign.
     (Example:
     ExcludeNodes ABCD1234CDEF5678ABCD1234CDEF5678ABCD1234, \{cc}, 255.254.0.0/8) +