Allow downstream libraries' documentation to work without feature flags

[madsmtm]

Running cargo doc --target=aarch64-linux-android in winit fails to build because we haven't selected an activity.

It would be nice if that wasn't the case, e.g. the library compiled in #[cfg(doc)] successfully without selecting an activity.

(I think this is also why currently, winit doesn't display any platform-specific documentation for Android on docs.rs).

[MarijnS95]

There is already a --cfg docsrs to steer some of this when we build this maybe it might help:

https://github.com/rust-mobile/android-activity/blob/main/android-activity/Cargo.toml#L62C27-L62C33

---

However:

Running cargo doc --target=aarch64-linux-android in winit fails to build because we haven't selected an activity.

That is strange though because there's already a && not(doc) there, to not panic if no activity is selected (but I guess it doesn't compile thereafter regardless, and we need to select a default?):

android-activity/android-activity/src/lib.rs

Lines 79 to 84 in 865cc6a

	#[cfg(all(
	not(any(feature = "game-activity", feature = "native-activity")),
	not(doc)
	))]
	compile_error!(
	r#"Either "game-activity" or "native-activity" must be enabled as features