Adding warning about depending on concrete implementation of Session#13884

oleg-andreyev wants to merge 2 commits intosymfony:3.4symfony/symfony-docs:3.4from

oleg-andreyev:flash-message-access-via-flashbag-interfaceoleg-andreyev/symfony-docs:flash-message-access-via-flashbag-interfaceCopy head branch name to clipboard