[3.12] gh-114913: Add newline to subprocess doc (GH-114941) (#114942)

*creationflags* is a separate topic from *startupinfo*.
Start sentence with 'If given', like previous sentence.
(cherry picked from commit 1183f1e6bfba06ae6c8ea362f96e977bc288e627)

Co-authored-by: Terry Jan Reedy <tjreedy@udel.edu>