docs/manual: add ordering of kconfig options' attributes

docs/manual: add ordering of kconfig options' attributes

We want to ensure all packages follow the same coding style, so we add a
blurb about the order of kconfig options' attributes:

  - first, the type and the prompt
  - the default value if needed
  - then dependencies of the 'depends on' category
  - followed by dependencies of the 'select' form
  - with eventually the help text

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>