make.bash: document CC_FOR_${GOOS}_${GOARCH}
make.bash: document CC_FOR_${GOOS}_${GOARCH}

Fixes #51306

Change-Id: I5989d86fe5ac4d02793b2ecb00c549d9586763da
GitHub-Last-Rev: 6b0f6bee434219a2c502e8b5630fbe74ca25024a
GitHub-Pull-Request: golang/go#51379
Reviewed-on: https://go-review.googlesource.com/c/go/+/388176
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
Run-TryBot: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Trust: Cherry Mui <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>

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