Change in coreboot[master]: docs/coding_style: Clarify use of GCC extensions

[Julius Werner (Code Review)]

Attention is currently required from: Arthur Heymans, Hung-Te Lin, Nico Huber, Paul Menzel.
Julius Werner has posted comments on this change. ( https://review.coreboot.org/c/coreboot/+/63660 )

Change subject: docs/coding_style: Clarify use of GCC extensions
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Patch Set 2:

(1 comment)

File Documentation/contributing/coding_style.md:

https://review.coreboot.org/c/coreboot/+/63660/comment/ca374833_9604a49e 
PS2, Line 994: "Extensions to the C Language Family" section of the GCC manual
> Make it a link?

As far as I'm aware, there's no way to link to any specific page on the "current" GCC version manual in a way that automatically stays up to date. I don't want to link to the 11.2 manual because this isn't supposed to be bound to only the extensions what were available at a certain time (although in practice they're not adding many new ones these days).