docs: point out that locals should be defined at the top of a block of code

docs: point out that locals should be defined at the top of a block of code

Although we have nothing in make syntax-check to enforce this, and
apparently there are places where it isn't the case (according to
Dan), we should discourage the practice of defining new variables in
the middle of a block of code.

https://www.redhat.com/archives/libvir-list/2020-July/msg00433.html

Signed-off-by: Laine Stump <laine@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Pavel Hrdina <phrdina@redhat.com>