[docs] [issue17987] test.support.captured_stderr, captured_stdin not documented

[Roundup Robot]

Roundup Robot added the comment:

New changeset af2416c2e27c by R David Murray in branch '3.3':
#17987: properly document support.captured_xxx.
http://hg.python.org/cpython/rev/af2416c2e27c

New changeset d0f7f1996001 by R David Murray in branch 'default':
Merge #17987: properly document support.captured_xxx.
http://hg.python.org/cpython/rev/d0f7f1996001

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nosy: +python-dev

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