doc: dts: improve rules for upstream bindings
doc: dts: improve rules for upstream bindings

Try to make the section which contains rules for upstream bindings
clearer. I'm having trouble getting people to do the right thing even
when referring to this section during code reviews, which makes me
think the docs need to be more explicit and contain more examples.

Clean up some section titles while I'm here.

Signed-off-by: Martí Bolívar <9d90ac4c7bf6a305f6bfd81a23c7859bc883380e@nordicsemi.no>
Signed-off-by: Gerard Marull-Paretas <76c1da08be458687d0066397d08e4f7ea9894377@teslabs.com>

galak/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,galak/zephyr,finikorg/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,finikorg/zephyr,galak/zephyr