fixed example to check if the session exists when getting flash messages to avoid starting sessions when not needed
added doctrine tags to reference
[#2571] Documented multiselect feature
Cleaned the FileSystem docs
Updated sidebar to use Debug component
Book/Controller : Minor tweak to gather 2 sentences

Factoring 2 sentences to make it easier to read.
Added documentation for the EqualConstraint
Added docs for NotEqualTo validator
Added docs for IdenticalTo validator
Added docs for NotIdenticalTo validator
[#2531] Documented dumpFile
Merge pull request #2591 from WouterJ/clean_filesystem_docs

Cleaned the FileSystem docs
Merge pull request #2597 from gregquat/patch-8

Book/Controller : Minor tweak to gather 2 sentences
Fix PHP syntax error in HttpKernel component working example

Extra comma after method argument removed.
[#2586] Documented setRedrawFrequenty()
Added docs for LessThan validator
Added docs for LessThanOrEqual validator
Added docs for GreaterThan validator
Added docs for GreaterThanOrEqual validator
documented Crawler::html()