runtime: cpu suboption for GODEBUG is not documented with the others for GODEBUG

[laboger]

I've seen mention of using GODEBUG=cpu to disable certain cpu capabilities in various issues. I expected to find it described in runtime/extern.go where the other GODEBUG suboptions are documented, but it is not.

[mknyszek]

Where do you suggest the documentation should go? (I agree it should be documented.)

[ianlancetaylor]

I think it pretty much has to go in the runtime package. Which is what @laboger suggests. It's a reasonable location, and where else could it go?

[martisch]

Its currently only documented at https://go.dev/doc/diagnostics. I agree we should add it to runtime/extern.go too.

[gopherbot]

Change https://go.dev/cl/423275 mentions this issue: runtime: document GODEBUG=cpu.* in runtime package