minor #13157 Add note about final classes (OskarStark)

This PR was squashed before being merged into the 3.4 branch.

Discussion
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Add note about final classes

![Screenshot 2020-02-17 08 56 05](https://user-images.githubusercontent.com/995707/74634030-57827100-5163-11ea-8276-2d6bac34a951.png)

Commits
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2743d6f Add note about final classes

Author: OskarStark

service_container/lazy_services.rst

@@ -23,6 +23,10 @@ Configuring lazy services is one answer to this. With a lazy service, a
 just like the ``mailer``, except that the ``mailer`` isn't actually instantiated
 until you interact with the proxy in some way.
 
+.. caution::
+
+    Lazy services do not support `final`_ classes.
+
 Installation
 ------------
 
@@ -106,3 +110,4 @@ in the `documentation of ProxyManager`_.
 .. _`proxy`: https://en.wikipedia.org/wiki/Proxy_pattern
 .. _`documentation of ProxyManager`: https://github.com/Ocramius/ProxyManager/blob/master/docs/lazy-loading-value-holder.md
 .. _`ocramius/proxy-manager`: https://github.com/Ocramius/ProxyManager
+.. _`final`: https://www.php.net/manual/en/language.oop5.final.php