minor symfony#12599 [Security] Minor tweak in the impersonation explanation (javiereguiluz)

This PR was squashed before being merged into the 4.3 branch (closes symfony#12599).

Discussion
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[Security] Minor tweak in the impersonation explanation

It's a minor change, but in the original text I don't know if it was clear that impersonated users keep their roles and Symfony adds a new one and doesn't replace the original roles.

Commits
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ed685a9 [Security] Minor tweak in the impersonation explanation

Author: OskarStark

security/impersonating_user.rst

@@ -85,9 +85,9 @@ role to the users that need it.
 Knowing When Impersonation Is Active
 ------------------------------------
 
-During impersonation, the user is provided with a special role called
-``ROLE_PREVIOUS_ADMIN``. In a template, for instance, this role can be used
-to show a link to exit impersonation:
+When a user is being impersonated, Symfony grants them a special role called
+``ROLE_PREVIOUS_ADMIN`` (in addition to the roles the user may have). Use this
+special role, for instance, to show a link to exit impersonation in a template:
 
 .. code-block:: html+twig