runtime: update docs for GOMAXPROCS
runtime: update docs for GOMAXPROCS

https://github.com/golang/go/blob/master/doc/effective_go.html#L3211 is used to update the docs comment for `GOMAXPROCS` function.

Fixes #41275

Change-Id: I39f58e93a267c6e9f3ac6638ed51acbe5284ada2
GitHub-Last-Rev: e45c8ac5873979397d747838fd8d41e252aec489
GitHub-Pull-Request: golang/go#41276
Reviewed-on: https://go-review.googlesource.com/c/go/+/253537
Reviewed-by: Keith Randall <8c99c3a9284e493be632950b84cd789d08ed3e9d@golang.org>

nawawi/go,cloudflare/go,CAFxX/go,cloudflare/go,golang/go,cloudflare/go,golang/go,golang/go,nawawi/go,CAFxX/go,golang/go,golang/go,nawawi/go,cloudflare/go,nawawi/go,nawawi/go,CAFxX/go,cloudflare/go,nawawi/go,golang/go,cloudflare/go,CAFxX/go,CAFxX/go,CAFxX/go,nawawi/go,CAFxX/go,cloudflare/go,CAFxX/go,golang/go,golang/go,nawawi/go,cloudflare/go