rsl.adoc: DELETE INDICATION is implemented since Feb '18
rsl.adoc: DELETE INDICATION is implemented since Feb '18

In the following commit, we introduced transmitting the
RSL DELETE INDICATION on AGCH overflow:

	commit 19da7fdea8912831b19d9aeed51cd2226f342283
	Author: Harald Welte <laforge@gnumonks.org>
	Date:   Sat Feb 24 04:32:29 2018 +0100

So let's sync the manual with the code.

Change-Id: I988778bdb83271355dc11b1a30a59e1a5dba5fb2
Related: OS#2990

osmocom/osmo-bts,osmocom/osmo-bts,osmocom/osmo-bts