Adding MigratingSessionHandler docs#9496

Merged

javiereguiluz merged 5 commits intosymfony:4.1from

AmaraLiving:add-migrating-session-handler

May 24, 2018

Commits on Mar 26, 2018

Adding MigratingSessionHandler docs
rossmotley
committed

Commits on May 17, 2018

Commits on May 22, 2018

Reworded and simplified
javiereguiluz
authored

Commits on May 24, 2018

Added the versionadded directive
javiereguiluz
authored