Docs: Section on local variables

[bearophile]

Nick Sabalausky:

> But it *is*, *objectively*, a downside of D versus C#.
> Disputing that with "But D gets rid of garbage initial values!" 
> is nothing but an irrelevant strawman.

This is right, and D people seem to forget it often. So I think 
we'll have to keep reminding this fact forever to D 
programmers/designers, in D.learn too.

Bye,
bearophile