Document the "replication" option in StartupMessage.

It is documented in the Streaming Replication Protocol section, but was
missing from the list of options in StartupMessage description.

doc/src/sgml/protocol.sgml

@@ -5138,6 +5138,21 @@ StartupMessage (F)
 </para>
 </listitem>
 </varlistentry>
+<varlistentry>
+<term>
+                <literal>replication</>
+</term>
+<listitem>
+<para>
+                Used to connect in streaming replication mode, where a small
+                set of replication commands can be issued instead of SQL
+                statements. Value can be <literal>true</>,
+                <literal>false</>, or  <literal>database</>, and the default
+                is <literal>false</>. See
+                <xref linkend="protocol-replication"> for details.
+</para>
+</listitem>
+</varlistentry>
 </variablelist>
 
                 In addition to the above, any run-time parameter that can be