ARROW-2673: [Python] Add documentation + docstring for ARROW-2661
ARROW-2673: [Python] Add documentation + docstring for ARROW-2661

Adding extra_conf arg documentation to the docstring

Author: zeroshade <mtopol@factset.com>

Closes #2893 from zeroshade/python/confdoc and squashes the following commits:

60bc9b0d4 <zeroshade> oops, line too long
537cc975f <zeroshade> Fixing documentation formatting
d86fe7a27 <zeroshade> ARROW-2673:  Add documentation + docstring for ARROW-2661

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