Added a caution about failing cache warmers

Author: javiereguiluz

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
 ~~~~~~~~~~~~~~~~~~