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ARROW-11665: [C++][Python] Improve docstrings for decimal and union types

Also add Python factory functions `pa.sparse_union` and `pa.dense_union`.

Closes #9560 from pitrou/ARROW-11665-py-decimal-doc

Authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Antoine Pitrou <antoine@python.org>