minor #949 Make clear to use Session BEFORE app->boot (sebastian-marinescu)

This PR was squashed before being merged into the 2.0.x-dev branch (closes #949).

Discussion
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Make clear to use Session BEFORE app->boot

It wasn't clear to me the session needs to be there before you boot.
I 'wasted' a great amount of time finding that out.
Please clarify for upcoming geniuses like me.

Commits
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5ddcc8b3 Make clear to use Session BEFORE app->boot

doc/providers/security.rst

@@ -64,6 +64,11 @@ Registering
             "symfony/security": "~2.3"
         }
 
+.. caution::
+
+    If you're using a form to authenticate users, you need to enable
+    ``SessionServiceProvider``.
+
 .. caution::
 
     The security features are only available after the Application has been
@@ -72,11 +77,6 @@ Registering
 
         $application->boot();
 
-.. caution::
-
-    If you're using a form to authenticate users, you need to enable
-    ``SessionServiceProvider``.
-
 Usage
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