arrow

MINOR: [Docs] document Float16 support in C++ (#38494)

### Rationale for this change

There is, at least, partial halffloat support in Arrow C++. A reason to not merge this is that there is still quite a bit casting work to be completed:
* https://github.com/apache/arrow/issues/20213
* https://github.com/apache/arrow/issues/32802

Authored-by: anjakefala <anja@voltrondata.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>