minor #12818 [Monolog] Add a caution note about Symfony Mailer and Monolog (javiereguiluz)

This PR was merged into the 4.3 branch.

Discussion
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[Monolog] Add a caution note about Symfony Mailer and Monolog

We can't fix #12758 properly until this is fixed in symfony/symfony#33456  but meanwhile we can display a caution message to avoid confusion.

Commits
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82119af [Monolog] Add a caution note about Symfony Mailer and Monolog

Author: OskarStark

logging/monolog_email.rst

@@ -4,6 +4,11 @@
 How to Configure Monolog to Email Errors
 ========================================
 
+.. caution::
+
+    This feature is not compatible yet with the new :doc:`Symfony mailer </mailer>`,
+    so it requires using SwiftMailer.
+
 `Monolog`_ can be configured to send an email when an error occurs with an
 application. The configuration for this requires a few nested handlers
 in order to avoid receiving too many emails. This configuration looks