docs/ubinascii: Document non-standard "sep" argument to hexlify().

docs/library/ubinascii.rst

@@ -10,10 +10,16 @@ encodings of it in ASCII form (in both directions).
 Functions
 ---------
 
-.. function:: hexlify(data)
+.. function:: hexlify(data, [sep])
 
    Convert binary data to hexadecimal representation. Return bytes string.
 
+   .. admonition:: Difference to CPython
+      :class: attention
+
+      If additional argument, `sep` is supplied, it is used as a seperator
+      between hexadecimal values.
+
 .. function:: unhexlify(data)
 
    Convert hexadecimal data to binary representation. Return bytes string.