doc: Add timeline as valid recovery target in standby.signal documentation

The documentation states that no target settings will be used when
standby.signal is present, but this is not quite the case since
recovery_target_timeline is a valid recovery target for a standby.

Update the documentation with this exception.

Author: David Steele <david@pgmasters.net>
Discussion: https://www.postgresql.org/message-id/flat/e445616d-023e-a268-8aa1-67b8b335340c%40pgmasters.net

doc/src/sgml/config.sgml

@@ -3156,7 +3156,9 @@ include_dir 'conf.d'
      use parameters in both <xref
      linkend="runtime-config-wal-archive-recovery"/> and <xref
      linkend="runtime-config-replication-standby"/>.  Parameters from
-     <xref linkend="runtime-config-wal-recovery-target"/> will not be used.
+     <xref linkend="runtime-config-wal-recovery-target"/> will not be used,
+     with the exception of <xref linkend="guc-recovery-target-timeline"/>
+     which allows a timeline to be selected for recovery.
     </para>
 
     <para>