Refactor: scheduler: best practices when checking probe-before-action

Previously, given a probe and another action, order_first_probe_unneeded()
checked whether a probe-then-action ordering was *not* needed.

Since that is a bit confusing to read, flip the sense of its return value, and
rename it probe_needed_before_action(). Also, add a doxygen block, return bool
rather than gboolean, rename a variable to avoid confusing
"left-hand/right-hand" terminology, and improve comments and formatting.