Revert "fixed example to check if the session exists when getting flash messages to avoid starting sessions when not needed"

It seems that there may be something stored in the flash, but you never know, because the session isn't started, so you never check.

This reverts commit a5168ad2f46264508b93b60c4c21a21384244662.
Book/Doctrine Minor fix

Fixing bad copy/paste ("yaml" => "xml").
I'm checking/updating French doc so I see everything (or almost) ;)
removed unneeded configuration
Merge pull request #2637 from gregquat/patch-11

Book/Doctrine: Minor typo fix
Added documentation for GroupSequence
[Book/Form] minor fix, superfluous word
Fixed scheme in XML format
Typo in code example

SessionStorage should be NativeSessionStorage. Example fails to execute otherwise.
Improving the VCL examples
Fix doubled words
fix doubled words
removed unneeded block
fixed attribute name
add documentation for the use of __call with propertyAccess
Merge pull request #2631 from symfony/session-flash-revert

Revert "fixed example to check if the session exists when getting flash ...
Merge pull request #2641 from bamarni/patch-7

[Twig] removed unneeded configuration
reference/.../property_path : Minor fix, suggest to use 'mapped' option ...

... the explanation was not complete
Fixed actual symfony number
Merge pull request #2647 from ClementGautier/2.0

[WIP] Added documentation for GroupSequence
[#2647] Minor tweaks to new validation group sequence docs