tm: Update docs of fr_timer to match the implementation

- Use description found in https://sip-router.org/wiki/ref_manual/timers

(cherry picked from commit 1195252256858dc4419c179e5b72541018f5feab)

src/modules/tm/doc/params.xml

@@ -18,8 +18,12 @@
     <section id="tm.p.fr_timer">
 	<title><varname>fr_timer</varname> (integer)</title>
 	<para>
-	    Timer which hits if no final reply for a request or ACK for a
-	    negative INVITE reply arrives (in milliseconds).
+		This timer is used for all SIP requests. It hits if no reply for an
+		request INVITE request or other request has been received
+		within (F in milliseconds). If a provisional reply is received for an
+		INVITE (any 1xx), then the fr_inv_timer will be used instead. And if
+		no replies (at all) for an INVITE are received before fr_timer hits,
+		the transaction is terminated with a 408 in failure route.
 	</para>
 	<para>
 	    Default value is 30000 ms (30 seconds).