Clarify filtering out by multiple values.

[jbothma]

While the implementation matches the logic in the documentation, I found it a bit unclear why my filter wasn't working.

I figured I'd make it really explicit for next time and to help others.

[coveralls]

Pull Request Test Coverage Report for Build 999

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 18 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-8.8%) to 17.897%

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Files with Coverage Reduction	New Missed Lines	%
/home/travis/virtualenv/python3.6.3/lib/python3.6/distutils/init.py	18	49.32%

Totals
Change from base Build 993:	-8.8%
Covered Lines:	1663
Relevant Lines:	9292

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💛 - Coveralls

[akariv]

Thanks @jbothma , it's indeed more clear now.