minor #1432 Update session.rst to explain handlers (Shruubi)
minor #1432 Update session.rst to explain handlers (Shruubi)

This PR was merged into the 2.1.x-dev branch.

Discussion
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Update session.rst to explain handlers

Expand the documentation to explain how to use different session providers.

Commits
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a925171 Update session.rst to explain handlers

romainneutron/Silex,gencer/Silex,romainneutron/Silex,silexphp/Silex,romainneutron/Silex