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
diff --git a/doc/manuals/chapters/configuration.adoc b/doc/manuals/chapters/configuration.adoc
index 177c096..c50f635 100644
--- a/doc/manuals/chapters/configuration.adoc
+++ b/doc/manuals/chapters/configuration.adoc
@@ -91,6 +91,11 @@
 trunk. However, it is important that the user assigns one of the E1 line numbers
 that were configured under the e1_input node.
 
+NOTE: The endpoint name that is used on MGCP level will include the trunk number,
+not the E1 line number. For simplicity (and compatibility with OsmoBSC) it is
+recommended to use equal numbers for trunk and E1 line. However, if required any
+E1 line can be mapped flexible on any trunk as long as the mapping is bijective.
+
 .Example: A typical configuration with one E1 trunk
 ----
 e1_input