Doc: libstonithd: Document what dlm uses

Similar to the many "sbd ... uses this" comments, to ensure that we
don't delete anything that dlm depends on.

Eventually dlm might use libpacemaker functions or something more sane.
However, we'll have to keep these helpers public for a while to support
old dlm versions.

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>