doc: Spell checking

doc/src/sgml/backup.sgml

doc/src/sgml/brin.sgml

doc/src/sgml/catalogs.sgml

doc/src/sgml/client-auth.sgml

doc/src/sgml/config.sgml

doc/src/sgml/custom-scan.sgml

doc/src/sgml/ddl.sgml

doc/src/sgml/fdwhandler.sgml

doc/src/sgml/func.sgml

doc/src/sgml/indices.sgml

doc/src/sgml/libpq.sgml

doc/src/sgml/logicaldecoding.sgml

doc/src/sgml/maintenance.sgml

doc/src/sgml/manage-ag.sgml

doc/src/sgml/plpgsql.sgml

doc/src/sgml/protocol.sgml

doc/src/sgml/queries.sgml

doc/src/sgml/recovery-config.sgml

doc/src/sgml/ref/alter_database.sgml

doc/src/sgml/ref/alter_table.sgml