Docs: Section on local variables

[bearophile]

Nick Sabalausky:

> Since implicit fallthrough is going away, that's more a problem 
> with a *missing* warning (in the second example) rather than an 
> erroneous warning.

Implicit fallthrough is going away, but Walter has decided to add 
it a special case, when the case is totally empty it's allowed. 
So both programs are correct (I don't love special cases, but 
here I think Walter doesn't want to cause too much D code 
disruption).

Bye,
bearophile