doc: minor improvements with respect to the term “ruleset”

doc: minor improvements with respect to the term “ruleset”

Statements are elements of rules. Non-terminal statement are in particular
passive with respect to their rules (and thus automatically with respect to the
whole ruleset).

In “Continue ruleset evaluation”, it’s not necessary to mention the ruleset as
it’s obvious that the evaluation of the current chain will be continued.

Signed-off-by: Christoph Anton Mitterer <mail@christoph.anton.mitterer.name>
Signed-off-by: Florian Westphal <fw@strlen.de>