tbf_dl: make preemptive retransmission optional

Since [1], OsmoPCU already starts to retransmit downlink blocks before
the MS has had a chance to receive them and/or send the related
acknowledgement in uplink. Make this optional with the new VTY option
"no dl-tbf-preemptive-retransmission".

[1] e25b5b91f60f20f61096bc6199a05b58ee6c6328 ("tbf: Only create dummy frames if necessary")
Related: OS#2408
Change-Id: Id08aed513d4033aa0d4324c6ce07cbb2852f2f92
diff --git a/doc/manuals/vty/osmo-pcu_vty_reference.xml b/doc/manuals/vty/osmo-pcu_vty_reference.xml
index cc8fffa..ea2d674 100644
--- a/doc/manuals/vty/osmo-pcu_vty_reference.xml
+++ b/doc/manuals/vty/osmo-pcu_vty_reference.xml
@@ -1807,6 +1807,17 @@
         <param name='dl-tbf-idle-time' doc='keep an idle DL TBF alive for the time given' />
       </params>
     </command>
+    <command id='dl-tbf-preemptive-retransmission'>
+      <params>
+        <param name='dl-tbf-preemptive-retransmission' doc='retransmit blocks even before the MS had a chance to receive them (better throughput, less readable traces) (enabled by default)' />
+      </params>
+    </command>
+    <command id='no dl-tbf-preemptive-retransmission'>
+      <params>
+        <param name='no' doc='Negate a command or set its defaults' />
+        <param name='dl-tbf-preemptive-retransmission' doc='retransmit blocks even before the MS had a chance to receive them (better throughput, less readable traces)' />
+      </params>
+    </command>
     <command id='ms-idle-time &lt;1-7200&gt;'>
       <params>
         <param name='ms-idle-time' doc='keep an idle MS object alive for the time given' />