doc: Add advice on updating checkpoint_segments to max_wal_size

with suggestion from Michael Paquier

doc/src/sgml/release-9.5.sgml

@@ -152,6 +152,22 @@
      </para>
     </listitem>
 
+    <listitem>
+     <para>
+      The configuration parameter <varname>checkpoint_segments</varname> has
+      been removed.  Its place is taken by the new
+      setting <xref linkend="guc-max-wal-size">.  If you had previously
+      tuned <varname>checkpoint_segments</varname>, the following formula will
+      give you an approximately equivalent setting:
+<programlisting>
+max_wal_size = (3 * checkpoint_segments) * 16MB
+</programlisting>
+      Note that the default setting for <varname>max_wal_size</varname> is
+      much higher than the default <varname>checkpoint_segments</varname> used
+      to be, so setting this might no longer be necessary.
+     </para>
+    </listitem>
+
     <listitem>
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 2015-07-28 [6087d95] Andres..: Remove ssl renegotiation support.