opt-in fns behind feature flags should be documented as such

[osa1]

Compare

• https://docs.rs/tokio/1.15.0/tokio/time/fn.timeout.html
• https://gtk-rs.org/gtk-rs-core/git/docs/cairo/struct.ImageSurface.html#method.write_to_png

Both of these are behind feature flags. In the first one which feature flag you need is documented. In the second one you need to search rest of the documentation for tips.

It would be helpful if gtk-rs packages documented required feature flags for function in the function documentation.

[sdroege]

That's because here the #[cfg] is on the whole module imported in lib.rs instead of per function: https://gtk-rs.org/gtk-rs-core/git/docs/src/cairo/lib.rs.html#145

Fixing that would require moving it down on each function (exclusively or additionally).