Fix documentation of ALTER TABLE for stored values

Author: Masahiko Sawada
Discussion: https://postgr.es/m/CAD21AoAA_gvZ002U6kovOHu0FsM7ieoCzdSqWBd7_KaQL0UMKg@mail.gmail.com

Author: michaelpq

doc/src/sgml/ref/alter_table.sgml

@@ -98,6 +98,7 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
   NULL |
   CHECK ( <replaceable class="parameter">expression</replaceable> ) [ NO INHERIT ] |
   DEFAULT <replaceable>default_expr</replaceable> |
+  GENERATED ALWAYS AS ( <replaceable>generation_expr</replaceable> ) STORED |
   GENERATED { ALWAYS | BY DEFAULT } AS IDENTITY [ ( <replaceable>sequence_options</replaceable> ) ] |
   UNIQUE <replaceable class="parameter">index_parameters</replaceable> |
   PRIMARY KEY <replaceable class="parameter">index_parameters</replaceable> |