doc: Fix generated column documentation

The old text still had an implicit reference to the virtual behavior,
which was not in the final patch.

Author: Tobias Bussmann <t.bussmann@gmx.net>

doc/src/sgml/ref/create_foreign_table.sgml

@@ -268,8 +268,8 @@ CHECK ( <replaceable class="parameter">expression</replaceable> ) [ NO INHERIT ]
     <listitem>
      <para>
       This clause creates the column as a <firstterm>generated
-      column</firstterm>.  The column cannot be written to, and when read it
-      will be computed from the specified expression.
+      column</firstterm>.  The column cannot be written to, and when read the
+      result of the specified expression will be returned.
      </para>
 
      <para>

doc/src/sgml/ref/create_table.sgml

@@ -813,8 +813,8 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
     <listitem>
      <para>
       This clause creates the column as a <firstterm>generated
-      column</firstterm>.  The column cannot be written to, and when read it
-      will be computed from the specified expression.
+      column</firstterm>.  The column cannot be written to, and when read the
+      result of the specified expression will be returned.
      </para>
 
      <para>