Add warning about DDOS to Htmx::trigger()

Background information

• confidential private issue: https://git.drupalcode.org/security/185098-drupal-security/-/work_items/1
  (included for reference. Please do not report access denied as an error.)

Problem/Motivation

Passing user input to Htmx::trigger() could be used for DDOS attacks if the trigger is set to a very low interval.
This is slightly mitigated by the fact HTMX waits for the current request to complete before starting the next.

It should be fairly obvious that passing user input to this method is a bad idea but we might still want to state that explicitly in the docblock.

Steps to reproduce

$userProvidedValue = 'every 0.001s';

// The browser will start spamming the URL.
Htmx::trigger('click', $userProvidedValue);

Proposed resolution

Add a warning to the docblock saying $triggerDefinition should not be provided by untrusted users.

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet