minor #6686 Update installation.rst (mgkimsal)

This PR was submitted for the 3.1 branch but it was merged into the 2.7 branch instead (closes #6686).

Discussion
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Update installation.rst

Reminder note to view specific documentation version when installing specific version.  Reference to installing LTS means example commands later on down in this same page will not work.

Commits
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c876100 Update installation.rst

setup.rst

@@ -89,6 +89,9 @@ optional second argument of the ``new`` command:
     # use the most recent 'lts' version (Long Term Support version)
     $ symfony new my_project_name lts
 
+Each version has its *own* documentation, which you can select on any documentation
+page.
+
 .. note::
 
     Read the :doc:`Symfony Release process </contributing/community/releases>`