Docs: Section on local variables

[bearophile]

Andrej Mitrovic:

> I think these features probably belong to some lint-type tool 
> and not the compiler.

On the contrary, I hope to see them implemented in the D 
front-end. I've seen again and again that people don't even use 
warnings, so I don't think many of them use lints. This means, 
tests should be built-in and active on default (this means I'd 
like D informational warnings to be active on default, and be 
disabled with a compiler switch!).

Bye,
bearophile