lib: add comments to exposed but internal symbols

lib: add comments to exposed but internal symbols

These two functions, despite being exposed by the shared library, are
not part of the public ABI.  They are only meant to be used by the
Python plugin provider, which is conceptually part of libbabeltrace2,
but implemented as a separate shared library.  Add comments to explain
that.

Change-Id: I704240070403c9dd5d75f15e107c00b011d8a5ef
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2407
Tested-by: jenkins <jenkins@lttng.org>