minor #16365 Explaining how to actually get the session from Request (ThomasLandauer)

This PR was submitted for the 5.3 branch but it was merged into the 5.4 branch instead.

Discussion
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Explaining how to actually get the session from `Request`

Question: Shouldn't `Request` be shown as preferred way (not `RequestStack`), since injecting `Request` is needed for forms anyway (whereas I've never needed to inject `RequestStack`)?

Commits
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9e6b105 Explaining how to actually get the session from `Request`

Author: javiereguiluz

session.rst

@@ -135,7 +135,7 @@ Check out the Symfony config reference to learn more about the other available
 Basic Usage
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-The session is available through the Request and the RequestStack.
+The session is available through ``->getSession()`` from the ``Request`` and the ``RequestStack``.
 Symfony provides a request_stack service that is injected in your services and
 controllers if you type-hint an argument with
 :class:`Symfony\\Component\\HttpFoundation\\RequestStack`::