Docs: Section on local variables

[bearophile]

Timon Gehr:

> I don't usually use warnings because of this issue:
> http://d.puremagic.com/issues/show_bug.cgi?id=6552

Unspecified falltrough is going to become an error, so it's not a 
good example of warning.

And if you are trying to say that warnings are not so good 
because they sometimes warn against something actually correct, 
that's a DMD bug, so again it's not a good example :-)

Regarding the OP topic I have opened:
http://d.puremagic.com/issues/show_bug.cgi?id=3960
http://d.puremagic.com/issues/show_bug.cgi?id=4694

Bye,
bearophile