docs/manual: size of tab in package description

docs/manual: size of tab in package description

Explicitly state that one tab counts for 8 columns in package
description, leaving 62 characters to the text itself.
Update the text and the example in the two places where the Config.in
format is described.
Also mention a newline is expected between the help text itself and the
upstream URL.

This blob can help developers to understand the expected formatting.
Also, it can be referenced by reviewers.

http://patchwork.ozlabs.org/patch/611289/
http://patchwork.ozlabs.org/patch/606866/
http://patchwork.ozlabs.org/patch/459960/

Signed-off-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>