docs: Add quick docs for ubinascii.

docs/library/index.rst

@@ -38,6 +38,7 @@ it will fallback to loading the built-in ``ujson`` module.
 .. toctree::
    :maxdepth: 1
 
+   ubinascii.rst
    uhashlib.rst
    uheapq.rst
    ujson.rst

docs/library/ubinascii.rst

@@ -0,0 +1,15 @@
+:mod:`ubinascii` -- binary/ASCII conversions
+============================================
+
+.. module:: ubinascii
+   :synopsis: binary/ASCII conversions
+
+This module implements conversions between binary data and various
+encodings of it in ASCII form (in both directions).
+
+Functions
+---------
+
+.. function:: hexlify(data)
+
+   Convert binary data to hexadecimal representation. Return bytes string.