Added some differences between Play 2.2 and sbt-less related to source maps.

Related discussion thread: https://groups.google.com/d/topic/play-framework/IO9BrHK4PN0/discussion

backport to 2.3.x
Merge pull request #3051 from v6ak/master

Add some differences between Play 2.2 and sbt-less related to source maps
Restructure session docs
Merge pull request #3055 from cchantep/session-tuto

Restructure session docs