ARROW-8320: [Format] Add clarification to CDataInterface.rst regardin…

…g memory alignment of buffers

This recommends at least a minimum alignment based on the type of data the buffer contains. For example, if the buffer contains `int32_t` data, then a minimum of 4-byte alignment is recommended.

If an Arrow implementation finds unaligned memory burdensome due to aliasing issues or undefined behavior, this also clarifies that they are not expected to support it.

Closes #6840 from wesm/ARROW-8320

Authored-by: Wes McKinney <wesm+git@apache.org>
Signed-off-by: Antoine Pitrou <antoine@python.org>

docs/source/format/CDataInterface.rst

@@ -440,6 +440,10 @@ It has the following fields:
    represent `length + offset` values encoded according to the
    :ref:`Columnar format specification <format_columnar>`.
 
+   It is recommended, but not required, that the memory addresses of the
+   buffers be aligned at least according to the type of primitive data that
+   they contain. Consumers MAY decide not to support unaligned memory.
+
    The pointer to the null bitmap buffer, if the data type specifies one,
    MAY be NULL only if :c:member:`ArrowArray.null_count` is 0.