Since b0c6762bc, every rule inside a referenced rule-set was evaluated
as if merged into the outer rule, which required tracking per-branch
group states and let outer rules and rule-set rules satisfy each
other's grouped conditions in both directions.
Restrict merging to the only designed case: a rule-set containing
exactly one non-inverted default rule is merged into the outer rule as
before. Any other rule-set now matches as an ordinary condition of the
outer rule: it matches when any of its rules matches on its own, and
its rules no longer exchange grouped match state with the outer rule
in either direction. Multiple referenced rule-sets keep OR semantics.
Flat address rule-sets such as generated geosite/geoip sets contain a
single default rule, so their behavior is unchanged. The group-state
set machinery is replaced by a single required/satisfied mask pair.
Also update the route and DNS rule docs.