Add documentation for strength parameter in Controlnet_img2img pipelines

[tlpss]

What does this PR do?

Adds documentation for strength parameter in Controlnet_img2img pipeline.
The docstring is copied from the Stable Diffusion img2img pipeline. A (pressumably) deprecated docstring was also updated with this same docstring for the Controlnet_SDXL_img2img pipeline.

Fixes #6784

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Who can review?

@yiyixuxu

[yiyixuxu]

thanks!

[HuggingFaceDocBuilderDev]

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