minor #7104 warning about using swiftmail with file spool and lazy loading (nykopol, javiereguiluz)

This PR was submitted for the master branch but it was merged into the 2.7 branch instead (closes #7104).

Discussion
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warning about using swiftmail with file spool and lazy loading

PR to fix #5596

Commits
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0a19f77 Fixed minor syntax issues and typos
ef3ab27 more precision on class name, and lazy service
f97db93 add warning about the limitation on using swiftmail with file spool and lazy loading

Author: xabbuh

email/spool.rst

@@ -141,3 +141,21 @@ You can also set the time limit in seconds:
 Of course you will not want to run this manually in reality. Instead, the
 console command should be triggered by a cron job or scheduled task and run
 at a regular interval.
+
+.. caution::
+
+    When you create a message with SwiftMailer, it generates a ``Swift_Message``
+    class. If the ``swiftmailer`` service is lazy loaded, it generates instead a
+    proxy class named ``Swift_Message_<someRandomCharacters>``.
+
+    If you use the memory spool, this change is transparent and has no impact.
+    But when using the filesystem spool, the message class is serialized in
+    a file with the randomized class name. The problem is that this random 
+    class name changes on every cache clear. So if you send a mail and then you
+    clear the cache, the message will not be unserializable.
+
+    On the next execution of ``swiftmailer:spool:send`` an error will raise because
+    the class ``Swift_Message_<someRandomCharacters>`` doesn't exist (anymore).
+
+    The solutions are either to use the memory spool or to load the 
+    ``swiftmailer`` service without ``lazy`` option (see :doc:`/service_container/lazy_services`).