Improve documentation on using cfg values with build scripts

[ehuss]

The docs don't clearly specify how to properly read cfg values in a build script:

Some things I think might be good to cover:

• Don't use cfg! or #[cfg] for target-related configs (only host-related to the build script itself).
• Don't examine the TARGET env for specific values, use the CARGO_CFG_* values instead.
• Some examples of how to properly handle comma-separated values (and maybe explicitly document which env vars have multiple values? or assume all of them do?).

This would be relevant on these pages:

• https://doc.rust-lang.org/cargo/reference/environment-variables.html#environment-variables-cargo-sets-for-build-scripts
• https://doc.rust-lang.org/cargo/reference/build-scripts.html
• https://doc.rust-lang.org/cargo/reference/build-script-examples.html

The environment variables page could contain a link to the build-script-examples page that shows how to use them.

Some discussion here: https://internals.rust-lang.org/t/futher-extensions-to-cfg-target-family-and-concerns-about-breakage/16313

[epage]

#12432 might also help with this

[epage]

I believe #14881 is a case of someone running into this confusion.