Mention that when alter rewrites a table, indexes are also rebuilt.

doc/src/sgml/ref/alter_table.sgml

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+$PostgreSQL: pgsql/doc/src/sgml/ref/alter_table.sgml,v 1.115 2010/06/24 14:57:21 momjian Exp $
 PostgreSQL documentation
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@@ -689,8 +689,8 @@ ALTER TABLE <replaceable class="PARAMETER">name</replaceable>
 
    <para>
     Adding a column with a non-null default or changing the type of an
-    existing column will require the entire table to be rewritten.  This
-    might take a significant amount of time for a large table; and it will
+    existing column will require the entire table and indexes to be rewritten.
+    This might take a significant amount of time for a large table; and it will
     temporarily require double the disk space.  Adding or removing a system
     <literal>oid</> column likewise requires rewriting the entire table.
    </para>