configuration: point out difference between trunk-nr and e1 line nr

When configuring osmo-mgw the user may choose an arbitrary trunk number
for the E1 trunk and sets a line number that must match the number of
the physical line that is used with the particular trunk. This is easy
to confuse, so lets add a note to the maual that mekes this clear.

Change-Id: I4b647a60d21cae99663a8258d6636ec8a7609d97
Related OS#5308

Change-Id: Ide27fda6d9ee2627bb544d21aa65161eace35a34
1 file changed