document canvas field on window in wasm

[mockersf]

Objective

• It's very hard to discover the canvas field to use an existing canvas in wasm. Improve it

Solution

• Add doc for the field and make it visible to rustdoc (https://doc.rust-lang.org/rustdoc/advanced-features.html#cfgdoc-documenting-platform-specific-or-feature-specific-information)

[bjorn3]

Maybe use #[doc(cfg(target_arch = "wasm32"))] too? That shows the cfg as part of the documentation. I'm not sure if it is stable already, but if not you could gate it to docs.rs.

[mockersf]

It's unstable: https://doc.rust-lang.org/rustdoc/unstable-features.html#doccfg-recording-what-platforms-or-features-are-required-for-code-to-be-present
Do you know if it or https://doc.rust-lang.org/rustdoc/unstable-features.html#doc_auto_cfg-automatically-generate-doccfg would be stabilised soon?

I'm not sure how to gate it for docs.rs, it seems it's only available through a var env in a build.rs script: https://docs.rs/about/builds#detecting-docsrs

[mockersf]

closed in favour of #4726 which remove the wasm32 limit on the field