minor symfony#51243 [Scheduler] Add some docs to explain the behavior (fabpot)

This PR was merged into the 6.4 branch.

Discussion
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[Scheduler] Add some docs to explain the behavior

| Q             | A
| ------------- | ---
| Branch?       | 6.4
| Bug fix?      | no
| New feature?  | no <!-- please update src/**/CHANGELOG.md files -->
| Deprecations? | no <!-- please update UPGRADE-*.md and src/**/CHANGELOG.md files -->
| Tickets       | n/a
| License       | MIT
| Doc PR        | n/a

Commits
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5aa5764 [Scheduler] Add some docs to explain the behavior

Author: fabpot

src/Symfony/Component/Scheduler/Trigger/TriggerInterface.php

@@ -16,5 +16,8 @@
  */
 interface TriggerInterface extends \Stringable
 {
+    /**
+     * Returns the next run date; if null is returned, this method won't be called again.
+     */
     public function getNextRunDate(\DateTimeImmutable $run): ?\DateTimeImmutable;
 }