Clarify constraints overhaul in user guide
Clarify constraints overhaul in user guide

Co-authored-by: Paul Moore <166cd9666f57883e93bcaf18256d094d703cedb3@gmail.com>

pradyunsg/pip,pfmoore/pip,pypa/pip,pypa/pip,sbidoul/pip,sbidoul/pip,pfmoore/pip,pradyunsg/pip