Doc: Recommend shlex.quote alongside pipes removal (#126570)

One of the most common reasons I see the old `pipes` module still in use
when porting to Python 3.13 is for the undocumented `pipes.quote`
function, which can easily be replaced with `shlex.quote`.  I think it's
worth specifically calling this out, since being directed to the
`subprocess` module would be confusing in this case.