[doc] ScalaHttpFilters should mention use of CSRFFilter

[wsargent]

This guy:

https://github.com/playframework/playframework/blob/master/framework/src/play-filters-helpers/src/main/scala/play/filters/csrf/csrf.scala

needs love here:

http://www.playframework.com/documentation/2.2.x/ScalaHttpFilters

[wsargent]

For example, if you're going to override doFilter with CSRFFilter, you need to call the apply after the case class automagic apply, which is not the norm:

object Global extends GlobalSettings {
  override def doFilter(action:EssentialAction) = CSRFFilter().apply(next = action)
}

[wsargent]

Closing this in favor of #1728