minor #21212 [Scheduler] Add warning about comma-separated weekdays in PeriodicalTrigger (wazum)

This PR was merged into the 6.4 branch.

Discussion
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[Scheduler] Add warning about comma-separated weekdays in `PeriodicalTrigger`

### Issue

symfony/symfony#60745

Users attempt to use comma-separated weekdays like "Monday, Thursday, Saturday" with RecurringMessage::every(), which causes silent failures due to PHP's DateInterval::createFromDateString()

### Solution
Added a caution note in the PeriodicalTrigger section warning users about this limitation and providing the correct alternative using cron expressions.

### Changes
- Added .. caution:: block in the Periodical Triggers section
- Shows the problematic usage pattern to avoid
- Provides working cron expression alternative: RecurringMessage::cron('5 12 * * 1,4,6', $message)
- Includes timezone handling example: RecurringMessage::cron('5 12 * * 1,4,6', $message, 'Europe/Warsaw')

Commits
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6a007d4 bug #60745 [Scheduler] Add warning about comma-separated weekdays in PeriodicalTrigger

Author: javiereguiluz

scheduler.rst

@@ -286,6 +286,16 @@ defined by PHP datetime functions::
     RecurringMessage::every('3 weeks', new Message());
     RecurringMessage::every('first Monday of next month', new Message());
 
+.. note::
+
+    Comma-separated weekdays (e.g., ``'Monday, Thursday, Saturday'``) are not supported
+    by the ``every()`` method. For multiple weekdays, use cron expressions instead:
+
+    .. code-block:: diff
+
+        - RecurringMessage::every('Monday, Thursday, Saturday', new Message());
+        + RecurringMessage::cron('5 12 * * 1,4,6', new Message());
+
 .. tip::
 
     You can also define periodic tasks using :ref:`the AsPeriodicTask attribute <scheduler-attributes-periodic-task>`.