document that lists of integers can be specified if encoding(octet) is used

This seems to be a good compromise: The API now strongly encourages
lists of characters, which are ideally suited to represent text, and
also allows lists of bytes if the encoding(octet) option is used.

src/prolog/lib/crypto.pl

@@ -10,6 +10,10 @@
 
    Especially for cryptographic applications, it as an advantage that
    using strings leaves little trace of what was processed in the system.
+
+   For predicates that accept an encoding/1 option to specify the encoding
+   of the input data, if encoding(octet) is used, then the input can also
+   be specified as a list of bytes, i.e., integers between 0 and 255.
 - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - */
 
 :- module(crypto,