minor #7374 Added a note about case-insensitiveness of service ids (javiereguiluz)

This PR was squashed before being merged into the master branch (closes #7374).

Discussion
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Added a note about case-insensitiveness of service ids

This fixes #7367. I propose to add just a small note because my guess it that the case-insensitive behavior is unused by 99.99% of developers and most of them are even unaware of it.

Commits
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0071fae Added a note about case-insensitiveness of service ids

Author: xabbuh

service_container.rst

@@ -154,6 +154,12 @@ later how you can configure a service that has multiple instances in the
     ``get()`` method to locate and retrieve the ``app.mailer`` service from
     the service container.
 
+.. caution::
+
+    Service identifiers are case-insensitive (``app.mailer`` and ``APP.Mailer``
+    for example refer to the same service). This behavior was deprecated in
+    Symfony 3.3 and it will no longer work in Symfony 4.0.
+
 .. _service-container-parameters:
 
 Service Parameters