[docs] [issue27528] Document that filterwarnings(message=...) matches the start of a message

[Roundup Robot]

Roundup Robot added the comment:

New changeset 739f638bb9f8 by Martin Panter in branch '2.7':
Issue #27528: Document and test warning messages must match at beginning
https://hg.python.org/cpython/rev/739f638bb9f8

New changeset 7571432200a2 by Martin Panter in branch '3.5':
Issue #27528: Document and test warning messages must match at beginning
https://hg.python.org/cpython/rev/7571432200a2

New changeset e0761e817deb by Martin Panter in branch 'default':
Issue #27528: Merge warning doc and test from 3.5
https://hg.python.org/cpython/rev/e0761e817deb

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nosy: +python-dev

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