man: clarify that ExecCondition= skip triggers SuccessAction=

man: clarify that ExecCondition= skip triggers SuccessAction=

Unit-level Condition…= / Assert…= directives that fail prevent activation
entirely, so SuccessAction= / FailureAction= never fire. An ExecCondition=
skip in [Service] runs as part of the activation transition itself, so
the unit goes active → inactive and SuccessAction= is honored.

This asymmetry has tripped users up; document it explicitly in both
systemd.service(5) (ExecCondition= section) and systemd.unit(5)
(SuccessAction= / FailureAction= section) with cross-references.

Fixes: https://github.com/systemd/systemd/issues/42035
Co-developed-by: Claude Opus 4.7 <noreply@anthropic.com>