Document why the fd tracker is ABI without a public header

The fd tracker feature is part of the ABI and used by liblttng-ust-fd.
However, some test code and documentation needs to be written before it
is exposed to users with a public header.

Change-Id: I601e11328426904c121ff6cd9668abbd83006f68
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>