Document why context provider is ABI without a public header

The context provider feature is part of the ABI and used by the Java jni
interface. However, some test code and documentation needs to be written
before it is exposed to users with a public header.

Change-Id: I24fcd5cf4a79e3f3c95654dd915dbe7c5380b66c
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>