minor #5018 Added a note about the class option of the services defined via factories (javiereguiluz)

This PR was merged into the 2.3 branch.

Discussion
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Added a note about the class option of the services defined via factories

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes
| Applies to    | 2.3+
| Fixed tickets | #4368

The original comment by @xabbuh in #4368 explained everything so well, that I decided to just copy it.

Commits
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2551322 Rewording
90956cc Minor rewording
477da91 Minor rewording
27ca372 Added a note about the class option of the services defined via factories

Author: weaverryan

components/dependency_injection/factories.rst

@@ -68,6 +68,14 @@ class:
 
         $container->setDefinition('newsletter_manager', $definition);
 
+.. note::
+
+    When using a factory to create services, the value chosen for the ``class``
+    option has no effect on the resulting service. The actual class name only
+    depends on the object that is returned by the factory. However, the configured
+    class name may be used by compiler passes and therefore should be set to a
+    sensible value.
+
 When you specify the class to use for the factory (via ``factory_class``)
 the method will be called statically. If the factory itself should be instantiated
 and the resulting object's method called, configure the factory itself as a service.