Clarify key-length terminology to match my intention

pep-0272.txt

@@ -130,9 +130,11 @@ Specification
 
         An integer value; the size of the keys required by this
         module, measured in bytes.  If key_size is None, then the
-        algorithm accepts arbitrary-length keys.  You cannot pass a
-        key of length 0 (that is, the null string '') as such a
-        variable-length key.
+        algorithm accepts variable-length keys.  This may mean the
+        module accepts keys of any random length, or that there are a
+        few different possible lengths, e.g. 16, 24, or 32 bytes.  You
+        cannot pass a key of length 0 (that is, the null string '') as
+        a variable-length key.
 
     Cipher objects should have two attributes:
 
@@ -204,6 +206,9 @@ Changes
     mode; added CTR and OFB feedback modes; clarified where numbers 
     are measured in bytes and where in bits.
 
+    2002-09: Clarified the discussion of key length by using
+    "variable-length keys" instead of "arbitrary-length".
+
 
 Acknowledgements