minor #7291 Reworded the user checker aliases explanation (javiereguiluz)

This PR was squashed before being merged into the 2.8 branch (closes #7291).

Discussion
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Reworded the user checker aliases explanation

The original explanation looked confusing to me. Let's see if others agree.

Commits
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de34b77 Reworded the user checker aliases explanation

Author: xabbuh

security/user_checkers.rst

@@ -211,8 +211,3 @@ It's possible to have a different user checker per firewall.
                 ),
             ),
         ));
-
-.. note::
-
-    Internally the user checkers are aliased per firewall. For ``secured_area``
-    the alias ``security.user_checker.secured_area`` would point to ``app.user_checker``.