document why ipa chopped ping fails with control interface

We cannot test the control interface with an IPA ping message.
The control interface only supports osmocom-specific extensions,
and the IPA ping message is not part of those extensions.
Other tests will have to be devised for the control interface.

Change-Id: Iae0f16394c78196de621c14b34d1b1905c0cb7c8
Related: OS#2010
1 file changed