Add feature requirement info to image loading docs

[bugsweeper]

Objective

• Add "Available on crate feature only." for docs of image format related types/functions
• Add warning "WARN bevy_render::texture::image: feature "" is not enabled" on load attempt
• Fixes Image loading docs are not clear enough about required feature flags #13468 .

Solution

• Added #[cfg(feature = "")] for types and warn!("feature "" is not enabled"); for ImageFormat enum conversions

Testing

ran reproducing example from issue #13468 and saw in logs
WARN bevy_render::texture::image: feature "exr" is not enabled

generated docs with command RUSTDOCFLAGS="-Zunstable-options --cfg=docsrs" cargo +nightly doc --workspace --all-features --no-deps --document-private-items --open and saw

that docs contain Available on crate feature <image format> only. marks

Migration Guide

Image format related entities are feature gated, if there are compilation errors about unknown names there are some of features in list (exr, hdr, basis-universal, png, dds, tga, jpeg, bmp, ktx2, webp and pnm) should be added.

[alice-i-cecile]

I generally really like it! Shouldn't we use module-level feature gates for e.g. ktx.rs?

[github-actions]

It looks like your PR is a breaking change, but you didn't provide a migration guide.

Could you add some context on what users should update when this change get released in a new version of Bevy?
It will be used to help writing the migration guide for the version. Putting it after a ## Migration Guide will help it get automatically picked up by our tooling.

[BD103]

I added the breaking change label because some enum variants of ImageFormat may no longer be available due to feature gates. (The same may be true for the modules, I'm not sure.)

[bugsweeper]

Shouldn't we use module-level feature gates for e.g. ktx.rs?

ktx2 module already has feature gate, but rust compiler isn't smart enough to mark all inner items of module with feature mark, that's why I marked explicitly

[alice-i-cecile]

Oh? That's surprising to me. What happens if it's ommitted? Do you have any references to bugs etc for this?

[bugsweeper]

What happens if it's ommitted?

Before my commit there already was attribute on ktx2 module and functions were not marked in docs. Now if attribute on module exists, but for function it was removed, then there is no mark in doc, if attribute on function exists, but for module it was removed, then mark in doc for function exists. I assumed that higher level attribute could reduce binary size, that is why i kept them both.

Do you have any references to bugs etc for this?

I didn't see any related issue earlier, and I didn't find it now. I just reacted to compiller behaviour. If I made an incorrect changes, I'm open to any corrections.

[alice-i-cecile]

Sounds good :) I'll file something for the rustdoc team reporting this!

[tychedelia]

Not sure I understand why logging versus a compiler error is preferred given this seems to be a bug but this is certainly an improvement in user feedback either way.