rsl.adoc: add info about RSL_IE_OSMO_REP_ACCH_CAP
rsl.adoc: add info about RSL_IE_OSMO_REP_ACCH_CAP

The RSL documentation should reflect some explanatory info about the
recently added RSL_IE_OSMO_REP_ACCH_CAP IE.

Depends: libosmocore I61ea6bf54ea90bd69b73ea0f0f3dc19a4214207b
Change-Id: I1d70846c2c184f7a189074c51137bc1f38fb3859
Related: OS#4796 SYS#5114

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