Introduce chapter trx_if.adoc and add it to OsmoTRX and OsmoBTS

This chapter defines the protocol used between osmo-trx and
osmo-bts-trx.

Most of the text comes originally from osmo-trx.git/README, as it's the
only known documentation of the protocol other than the code itself.

Change-Id: I56c418eef0f826ae1aadbed5b151fbed241c7885
diff --git a/doc/manuals/osmotrx-usermanual.adoc b/doc/manuals/osmotrx-usermanual.adoc
index 249a21c..3b2e991 100644
--- a/doc/manuals/osmotrx-usermanual.adoc
+++ b/doc/manuals/osmotrx-usermanual.adoc
@@ -23,6 +23,8 @@
 
 include::chapters/configuration.adoc[]
 
+include::../common/chapters/trx_if.adoc[]
+
 include::../common/chapters/port_numbers.adoc[]
 
 include::../common/chapters/bibliography.adoc[]