In section 6.8.1 Popen3 and Popen4 Objects, of the
current and development docs,  under attribute 'childerr':

"""
Where the standard error from the child process goes is
capturestderr was true for the constructor, or None.
"""

The 'is' should be 'if'.

I think it might also be an improvement to replace 'or'
with 'otherwise'.

If a rewriting is acceptable, I think 

"""
A file object that provides error output from the child
process, if
capturestderr was true for the constructor, otherwise None.
"""
improves uniformity with the descriptions of
'fromchild' and 'tochild'.

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Your rewrite is certainly much better than the existing text; checked 
in as rev. 1.20.  Thanks!