Change in coreboot[master]: docs: correct and rewrite documentation regarding n/c / unused pads

[Michael Niewöhner (Code Review)]

Attention is currently required from:  Felix Singer, Nico Huber, Matt DeVillier, Tim Wawrzynczak, Patrick Rudolph.
Michael Niewöhner has posted comments on this change. ( https://review.coreboot.org/c/coreboot/+/52139 )

Change subject: docs: correct and rewrite documentation regarding n/c / unused pads
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Patch Set 9:

(1 comment)

File Documentation/getting_started/gpio.md:

https://review.coreboot.org/c/coreboot/+/52139/comment/c36593e4_ef7c6d93 
PS5, Line 133: usually don't need
> Pads defaulting to native functions and being inputs have a internal pull. […]

yeah, basically I was right. There's an exception: bad board design regarding a few pads without internal pulls. I've added that