man: move description of *Action= modes to FailureAction=/SuccessAction=

man: move description of *Action= modes to FailureAction=/SuccessAction=

FailureAction=/SuccessAction= were added later then StartLimitAction=, so it
was easiest to refer to the existing description. But those two settings are
somewhat simpler (they just execute the action unconditionally) while
StartLimitAction= has additional timing and burst parameters, and they are
about to take on a more prominent role, so let's move the description of
allowed values.