usermanual: add chapter about mgcp endpoints

The manual does not say much about the endpoint naming, even though
osmo-mgw is oriented at RFC 3435 the endpoint names are specific to
osmo-mgw. In particular the E1/T1 endpoints and their overlapping need
some explaination

Change-Id: I16265eb667221959a69f33701d024bd8d9b22040
2 files changed