Attached is a patch that adds documentation for a few things that have existed in subprocess for a while without documentation.

The "startupinfo" parameter takes subprocess.STARTUPINFO object which takes a few different options for its attributes, but none of this was documented.

The "creationflags" parameter takes subprocess.CREATE_NEW_WINDOW or subprocess.CREATE_NEW_PROCESS_GROUP, but neither were documented as options or what the options mean.

You can indent the "attribute" directives and avoid to repeat "STARTUPINFO." before every attribute. If STD_INPUT_HANDLE and the other constants are attributes of STARTUPINFO they should be indented too, otherwise a line that says that subprocess also provides those at module level might be better.
Another short note at the beginning of the section that mentions that this class/attributes are available on Windows only would be good too.
There's also some trailing whitespace that should be removed.

New changeset 609ca9d71aba by Brian Curtin in branch '3.1':
Fix #11961. Document STARTUPINFO and creation flag options.
http://hg.python.org/cpython/rev/609ca9d71aba

New changeset f0092c611004 by Brian Curtin in branch '3.2':
Fix #11961. Document STARTUPINFO and creation flag options.
http://hg.python.org/cpython/rev/f0092c611004

New changeset f8fae22a1ff0 by Brian Curtin in branch 'default':
Fix #11961. Document STARTUPINFO and creation flag options.
http://hg.python.org/cpython/rev/f8fae22a1ff0

New changeset 9a31422649f1 by Brian Curtin in branch '2.7':
Fix #11961. Document STARTUPINFO and creation flag options.
http://hg.python.org/cpython/rev/9a31422649f1

Thanks for having a look, Ezio.