docs(flux2): clarify image= is reference conditioning, not img2img

[NazarKozak]

What does this PR do?

The image= parameter on Flux2Pipeline and Flux2KleinPipeline is currently described as a "starting point", which mirrors the SD/Flux.1 img2img wording:

Image, numpy array or tensor representing an image batch to be used as the starting point. […] It can also accept image latents as image, but if passing latents directly it is not encoded again.

Flux.2 actually encodes reference images as additional attention conditioning tokens — there's no noisy-latent init, and so no strength parameter. The existing wording is easy to misread when porting code from StableDiffusionImg2ImgPipeline, FluxImg2ImgPipeline, or FluxKontextInpaintPipeline, all of which legitimately accept strength.

We had a production serving stack pass strength= alongside image= based on the existing wording and crashed with:

TypeError: Flux2Pipeline.__call__() got an unexpected keyword argument 'strength'

Changes

• Rewrite the image= docstring on Flux2Pipeline.__call__ and Flux2KleinPipeline.__call__ to make it explicit that this is reference conditioning, not img2img, and that there is no strength knob.
• Add a short ## Reference conditioning vs. img2img callout to docs/source/en/api/pipelines/flux2.md distinguishing the two modes, mentioning Flux2KleinInpaintPipeline as the right pipeline if you actually do need a strength-style noisy-latent init.

Flux2KleinKVPipeline's docstring already correctly describes reference conditioning, so no change there. Flux2KleinInpaintPipeline does take strength and is left as-is.

Out of scope (happy to add as a follow-up)

A small defensive check inside __call__ that intercepts unknown img2img kwargs and raises a clear TypeError pointing at this section of the docs, e.g.:

if "strength" in kwargs:
    raise TypeError(
        "Flux2Pipeline does not support `strength`. Flux.2 uses reference "
        "conditioning via `image=`, not noisy-latent img2img. See "
        "https://huggingface.co/docs/diffusers/api/pipelines/flux2 ."
    )

Left out of this PR to keep it docs-only; happy to push as a second commit if maintainers prefer.

Behavioral change

None — docs only.

Fixes # (no related issue — surfaced in production)

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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Who can review?

Docs: @stevhliu @sayakpaul