doc: Document Kconfig extensions and Zephyr-specific behavior

Document the gsource (globbing source), rsource (relative source), and
grsource (globbing + relative source) statements.

Also add a separate section that documents Zephyr's non-standard
prefer-later-defaults Kconfig behavior. It was only mentioned in passing
before.

Piggyback: Move the note about .config assignments having no effect on
invisible symbols closer to the beginning of the 'Configuring invisible
Kconfig symbols' section. It makes more sense there.

Signed-off-by: Ulf Magnusson <ulfalizer@gmail.com>