Document `var` and special behaviour of toplevel definitions for each scope

[mkurz]

1. Document var

2. Not just for var, but for val and def:

@val val1 = @{ "value1" }
@if(secondCondition == 1) {
    @val1 // This will also print shadow_value1 even thought it will be defined afterwards
    @val val1 = @{ "shadow_value1" }
    @val1
}

This is because in twirl all @... = (which define a def and does not reassign a var), @var foo =... and @[lazy] val foo ... are moved to the top of the current (pseudo template) (code) block. This is just how twirl works and can not be changed without more or less totally rewriting twirl. So let's document that behaviour. Also see tests, there a lots of example for that.