manual: clarify Tested-by/Reviewed-by/Acked-by tags

This patch updates the manual with more clarified descriptions of tags
Tested-by, Reviewed-by, and Acked-by, as discussed on the Buildroot
developer days in February 2014.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Acked-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>