docs/ustruct: Document pack_into(), unpack_from().

docs/library/ustruct.rst

@@ -19,7 +19,19 @@ Functions
    Pack the values ``v1``, ``v2``, ... according to the format string ``fmt``.
    The return value is a bytes object encoding the values.
 
+.. function:: pack_into(fmt, buffer, offset, v1, v2, ...)
+
+   Pack the values `v1`, `v2`, ... according to the format string `fmt`
+   into a `buffer` starting at `offset`. `offset` may be negative to count
+   from the end of `buffer`.
+
 .. function:: unpack(fmt, data)
 
    Unpack from the ``data`` according to the format string ``fmt``.
    The return value is a tuple of the unpacked values.
+
+.. function:: unpack_from(fmt, data, offset=0)
+
+   Unpack from the `data` starting at `offset` according to the format string
+   `fmt`. `offset` may be negative to count from the end of `buffer`. The return
+   value is a tuple of the unpacked values.