[Python] Improve ergonomics of compute functions

>>> from pyarrow import compute as pc                                                                                                                                 
>>> pc.list_flatten                                                                                                                                                   
<function pyarrow.compute._simple_unary_function.<locals>.func(arg)>
>>> ?pc.list_flatten                                                                                                                                                  
Signature: pc.list_flatten(arg)
Docstring: <no docstring>
File:      ~/arrow/dev/python/pyarrow/compute.py
Type:      function
>>> help(pc.list_flatten)                                                                                                                                             
Help on function func in module pyarrow.compute:

func(arg)