doc: kconfig: Move extension docs from porting guide to user guide

The Kconfiglib Kconfig extensions are documented in the porting guide,
which is a pretty weird place to document them. Document them under a
new page in the Kconfig section of User and Developer Guides instead.

Also remove the section about the old Zephyr Kconfig behavior for
defaults (tried last-to-first instead of first-to-last). The behavior
was changed 16 months ago (and was undocumented before that), so it
might not be that useful to mention anymore.

Piggyback misc. small language cleanups and organization nits.

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