Document usage of arch=("$CARCH") in PKGBUILDs

A frequent issue especially with PKGBUILDs in the AUR seems to be package maintainers using arch=('any') even for packages that produce architecture-dependent binaries, because they want to make life easier for people using Archlinux on non-x86 CPU architectures, without having to research and maintain an exhaustive list of all possible CPU architecture strings.

I've found that using arch=("$CARCH") appears to solve this issue for CPU-dependent packages, so I'm left with two questions:

1. Can arch=("$CARCH") be safely used in PKGBUILDs, or is that considered to be some sort of "undefined behaviour"?
2. Would you be willing to accept a merge request that adds a hint/suggestion about arch=("$CARCH") to the PKGBUILD man page? (Regardless of what the answer to question 1 might be, I'd consider it beneficial for this to be explicitly documented in the man page.)