Proposed patch attached.

I verified that the tests included in the patch pass on 2.7, 3.2, 3.3, and default (replacing FileNotFoundError with OSError as necessary).  I also verified that the tests pass on Windows for default.

The patch also includes improvements to the parts of Popen()'s documentation that relate to the added tests.  The improvements simplify the documentation by more clearly separating the argument-specific documentation from the default/general behavior.  Previously, certain general information was under various argument-specific paragraphs which made the documentation less clear.