[APIC-280] DOC clarify table_columns param in civis.io functions

[gkcivis]

Clarify table_columns parameter per https://civisanalytics.atlassian.net/browse/APIC-280

[jacksonlee-civis]

@gkcivis These changes are great! The TravisCI builds failed because flake8 was upset:

civis/io/_tables.py:684:80: E501 line too long (82 > 79 characters)
civis/io/_tables.py:684:83: W291 trailing whitespace
civis/io/_tables.py:685:80: E501 line too long (80 > 79 characters)
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civis/io/_tables.py:686:80: E501 line too long (80 > 79 characters)
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civis/io/_tables.py:688:80: E501 line too long (80 > 79 characters)
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civis/io/_tables.py:689:80: E501 line too long (82 > 79 characters)
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civis/io/_tables.py:690:80: E501 line too long (80 > 79 characters)
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civis/io/_tables.py:691:74: W291 trailing whitespace
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civis/io/_tables.py:834:80: E501 line too long (82 > 79 characters)
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civis/io/_tables.py:835:80: E501 line too long (80 > 79 characters)
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civis/io/_tables.py:836:80: E501 line too long (80 > 79 characters)
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civis/io/_tables.py:838:80: E501 line too long (80 > 79 characters)
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civis/io/_tables.py:839:80: E501 line too long (82 > 79 characters)
civis/io/_tables.py:839:83: W291 trailing whitespace
civis/io/_tables.py:840:80: E501 line too long (80 > 79 characters)
civis/io/_tables.py:840:81: W291 trailing whitespace
civis/io/_tables.py:841:74: W291 trailing whitespace
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civis/io/_tables.py:970:80: E501 line too long (82 > 79 characters)
civis/io/_tables.py:970:83: W291 trailing whitespace
civis/io/_tables.py:971:80: E501 line too long (80 > 79 characters)
civis/io/_tables.py:971:81: W291 trailing whitespace
civis/io/_tables.py:972:80: E501 line too long (80 > 79 characters)
civis/io/_tables.py:973:79: W291 trailing whitespace
civis/io/_tables.py:974:80: E501 line too long (80 > 79 characters)
civis/io/_tables.py:974:81: W291 trailing whitespace
civis/io/_tables.py:975:80: E501 line too long (82 > 79 characters)
civis/io/_tables.py:975:83: W291 trailing whitespace
civis/io/_tables.py:976:80: E501 line too long (80 > 79 characters)
civis/io/_tables.py:976:81: W291 trailing whitespace
civis/io/_tables.py:977:74: W291 trailing whitespace
civis/io/_tables.py:979:59: W291 trailing whitespace

(Note to our future selves: This PR partially addresses #380, but we have to leave that GitHub issue open even after this PR is merged, because we have yet to address a related doc issue from the Platform API -- see the linked ticket from the current APIC-280)

[gkcivis]

Is there a way to re-run TravisCI on the latest commit?

[jacksonlee-civis]

LGTM!

The TravisCI builds ran eventually 🐢 and finished with no errors. We should be good now!