OstreeManager: Add comments about Secondary functionality.
OstreeManager: Add comments about Secondary functionality.

Do not be thrown off as I was about Secondaries being able to use
getCurrent() due to calling functions with the word "Primary" in the
name. They will work fine on Secondaries since Secondaries only know
about themselves.

Signed-off-by: Patrick Vacek <a38520cbbe2e974bc8b7ea86877dd3c7115ca2d7@gmail.com>

advancedtelematic/aktualizr,advancedtelematic/aktualizr,advancedtelematic/sota_client_cpp,advancedtelematic/sota_client_cpp,advancedtelematic/aktualizr,advancedtelematic/aktualizr