Mention that triggers are guaranteed to throw an error or log it

[TarantoolBot]

Related dev. issue(s): tarantool/tarantool#9309

Product: Tarantool
Since: 3.1
Root document: not yet created (#3988)
SME: @ drewdzzz

Details

Now all triggers are either have a direct impact on the execution flow
(e.g. close a connection or throw an error) or print a message to error
log when they throw an error.

We have a separate entry for triggers in Tarantool documentation, which
lists the properties of the triggers - I think it's a perfect place for
this new property.
Requested by @ drewdzzz in tarantool/tarantool@2adfc66.