Error on missing web_sys_unstable_apis

[rukai]

closes #5095

I thought about this some more and I think we can and should just give the user a helpful error when they are missing the unstable cfg.
With this we no longer need to take up prime documentation real estate with it.
I imagine resources like sotrh's learn wgpu will push users towards using wgpu = { .. default-features = false, features = ["webgl"] } to make it easy for users to run on the web and then this error will catch the user when they get that slightly wrong.

Testing

I created a seperate crate that depends on wgpu and tested the following cases:
This compiles successfully:

wgpu = {version = "0.19", path = "", features = ["webgl"], default-features = false}

This compiles successfully:

wgpu = {version = "0.19", path = "", default-features = false}

This fails to compile with the new error:

wgpu = {version = "0.19", path = "" }

Checklist

• Run cargo fmt.
• Run cargo clippy. If applicable, add:
  • --target wasm32-unknown-unknown
  • --target wasm32-unknown-emscripten
• Run cargo xtask test to run tests.
• Add change to CHANGELOG.md. See simple instructions inside file.

[rukai]

I will investigate the CI failure.

[rukai]

cargo doc --target wasm32-unknown-unknown was failing in CI due to triggering the compile failure.
I resolved it by setting rustdocflags the same as we set for rustflags: https://doc.rust-lang.org/cargo/reference/config.html#buildrustdocflags
This fixed the issue locally.

Then I had to set RUSTDOCFLAGS env var in the CI script since it had a custom value that was overwriting rustdocflags config.
This fixed the issue in CI.

Then finally I changed the compile_error message to include setting rustdocflags since if we need to set it to get docs working then we should recommend that users set it too.

[cwfitzgerald]

This is great and a really novel solution I hadn't even considered!