docs/ustruct: Describe supported type codes.

docs/library/ustruct.rst

@@ -7,6 +7,12 @@
 See `Python struct <https://docs.python.org/3/library/struct.html>`_ for more
 information.
 
+Supported size/byte order prefixes: ``@``, ``<``, ``>``, ``!``.
+
+Supported format codes: ``b``, ``B``, ``h``, ``H``, ``i``, ``I``, ``l``,
+``L``, ``q``, ``Q``, ``s``, ``P``, ``f``, ``d`` (the latter 2 depending
+on the floating-point support).
+
 Functions
 ---------