lib: add comment to bt_plugin_so_create_all_from_static

lib: add comment to bt_plugin_so_create_all_from_static

This symbol would normally not be exposed, but it is used from the
Python plugin provider, which is conceptually part of libbabeltrace2,
but implemented as a separate shared library.  Add a comment to explain
that.

Change-Id: Ie6e14edc30489ab6f8c6aadba32a3ceb9726e3b0
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2406
Tested-by: jenkins <jenkins@lttng.org>