doc: apply an rssi-offset of 28 by default.

Set an rssi offset of 28 in the example configs to make sure that the
power control loop gets RSSI values that match at least half way the
reality when the 1800 Mhz band is used. For other bands the value will
be different (See also related osmocom ticket)

Change-Id: I62725fe454f54e2c7cb7550dadb1e6fc94337d78
Related: OS#4468
diff --git a/doc/examples/osmo-trx-uhd/osmo-trx-usrp_b200.cfg b/doc/examples/osmo-trx-uhd/osmo-trx-usrp_b200.cfg
index b851211..2905ed7 100644
--- a/doc/examples/osmo-trx-uhd/osmo-trx-usrp_b200.cfg
+++ b/doc/examples/osmo-trx-uhd/osmo-trx-usrp_b200.cfg
@@ -13,6 +13,9 @@
  bind-ip 127.0.0.1
  remote-ip 127.0.0.1
  egprs disable
+ ! 28 dB offset below is valid only for the B2xx in 1800 MHz band, see
+ ! https://osmocom.org/issues/4468 for more details
+ rssi-offset 28.000000
  tx-sps 4
  rx-sps 4
  clock-ref external