[book] Improve PostgreSQL configuration section * Do not advice for specific PostgreSQL version (point to meta-packages if available in Linux distros); * First mention packages from Linux distros, put the link to official postgres documentation at the end; Closes #2725302.

if not rql('CWConstraintType X WHERE X name "RQLUniqueConstraint"',
           ask_confirm=False):
    rql('INSERT CWConstraintType X: X name "RQLUniqueConstraint"',
        ask_confirm=False)