[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 repo.system_source.dbdriver == 'postgres':
    sql('ALTER TABLE appears ADD COLUMN weight float')
    sql('UPDATE appears SET weight=1.0 ')