extmod/modubinascii: Update code, docs for hexlify now CPython has sep.

Since CPython 3.8 the optional "sep" argument to hexlify is officially
supported, so update comments in the code and the docs to reflect this.

Signed-off-by: Damien George <damien@micropython.org>

docs/library/ubinascii.rst

@@ -14,13 +14,11 @@ Functions
 
 .. function:: hexlify(data, [sep])
 
-   Convert binary data to hexadecimal representation. Returns bytes string.
+   Convert the bytes in the *data* object to a hexadecimal representation.
+   Returns a bytes object.
 
-   .. admonition:: Difference to CPython
-      :class: attention
-
-      If additional argument, *sep* is supplied, it is used as a separator
-      between hexadecimal values.
+   If the additional argument *sep* is supplied it is used as a separator
+   between hexadecimal values.
 
 .. function:: unhexlify(data)
 

extmod/modubinascii.c

@@ -34,8 +34,8 @@
 #if MICROPY_PY_UBINASCII
 
 STATIC mp_obj_t mod_binascii_hexlify(size_t n_args, const mp_obj_t *args) {
-    // Second argument is for an extension to allow a separator to be used
-    // between values.
+    // First argument is the data to convert.
+    // Second argument is an optional separator to be used between values.
     const char *sep = NULL;
     mp_buffer_info_t bufinfo;
     mp_get_buffer_raise(args[0], &bufinfo, MP_BUFFER_READ);