Kconfig: Document the new prefer-earlier-defaults behavior

Update the documentation to say that earlier defaults are preferred,
while explaining Kconfig.defconfig files.

Keep the section that covered the Zephyr prefer-later-defaults patch as
documentation for the behavior having changed. It will be linked in a
warning later.

Also add documentation explaining how to configure choices, including
using multiple definition locations to change a choice 'default'.
Document that 'range's can be added/overriden as well. This works
reliably now.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>