manuals/abis/rsl.adoc: rework Channel Number description
manuals/abis/rsl.adoc: rework Channel Number description

Change-Id: I3013c6b2c38db33d7d80e174b3d4910f8b01963d
Related: SYS#4895, OS#4941

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