Mention change of width of values generated by SERIAL sequences

This changed during pg10 development, but had not been documented.
Co-authored-by: Jonathan S. Katz <jkatz@postgresql.org>
Discussion: https://postgr.es/m/20180828163408.vl44nwetdybwffyk@alvherre.pgsql

doc/src/sgml/release-10.sgml

@@ -4716,6 +4716,13 @@ Branch: REL_10_STABLE [5159626af] 2017-11-03 14:14:16 -0400
       more compatible with existing code.
      </para>
 
+     <para>
+      Also, sequences created for <literal>SERIAL</literal> columns now generate
+      positive 32-bit wide values, whereas previous versions generated 64-bit
+      wide values.  This has no visible effect if the values are only stored in
+      a column.
+     </para>
+
      <para>
       The output of <application>psql</application>'s <command>\d</command> command for a
       sequence has been redesigned, too.