tweak API version comments

Clarify LIBVERSION comments in TODO-RELEASE and reference Makefile.am comments
to look there.

Change-Id: I4edf786fde085f612f8a04972c55175a080ac65f
diff --git a/src/libosmo-mgcp-client/Makefile.am b/src/libosmo-mgcp-client/Makefile.am
index e644862..b17477a 100644
--- a/src/libosmo-mgcp-client/Makefile.am
+++ b/src/libosmo-mgcp-client/Makefile.am
@@ -19,9 +19,8 @@
 	$(COVERAGE_LDFLAGS) \
 	$(NULL)
 
-# This is _NOT_ the library release version, it's an API version.
-# Please read Chapter 6 "Library interface versions" of the libtool
-# documentation before making any modification
+# This is not at all related to the release version, but a range of supported
+# API versions. Read TODO_RELEASE in the source tree's root!
 MGCP_CLIENT_LIBVERSION=0:0:0
 
 lib_LTLIBRARIES = \