Clarify usage patterns spicy::decline_input() vs throw

[awelzel]

See discussion below - I'm uncertain if this is a Spicy or Zeek issue, putting it in Spicy for now.

zeek/zeek#3277 (comment)_

Chatting with @bbannier in referenced PR, it would be good to clarify usages of spicy::decline_input() vs throw. With Zeek's integration, using spicy::decline_input() within %error results in two analyzer_violation_info() events. Further, the documentation suggests to follow a spicy::decline_input() with throw to abort parsing, resulting in two violations, too, and then one could just throw in the first place.

Generally wondering: Is spicy::decline_input() for edge-cases? Doesn't throw provide the more expected behavior for most use-cases? (as being similar to &requires)