minor #6215 Added a caution about failing cache warmers (javiereguiluz)

This PR was merged into the 2.3 branch.

Discussion
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Added a caution about failing cache warmers

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes
| Applies to    | all
| Fixed tickets | -

This was asked for in symfony/symfony#11526

Commits
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dc62504 Added a caution about failing cache warmers

Author: xabbuh

reference/dic_tags.rst

@@ -456,6 +456,13 @@ tag:
     The ``priority`` value is optional and defaults to 0. The higher the
     priority, the sooner it gets executed.
 
+.. caution::
+
+    If your cache warmer fails its execution because of any exception, Symfony
+    won't try to execute it again for the next requests. Therefore, your
+    application and/or bundles should be prepared for when the contents
+    generated by the cache warmer are not available.
+
 Core Cache Warmers
 ~~~~~~~~~~~~~~~~~~