VaeImageProcessor.blur() docstring is missing blur_factor and has a copy-pasted description

`VaeImageProcessor.blur()`'s docstring is missing the `blur_factor` parameter entirely, and its
`Returns`/description text appears to be copy-pasted from a different method — it currently
describes converting an image to grayscale, not blurring it.

**Location:** `src/diffusers/image_processor.py`, `VaeImageProcessor.blur()`, around line 271.

**Proposed fix:** add the missing `blur_factor` entry to the `Args` section and correct the
description to accurately describe the blur behavior, matching the style of nearby docstrings
in the same file (1-3 sentences, no more).

I have a small, single-file fix ready and would like to open a PR for this if a maintainer can
confirm it's welcome — happy to follow whatever review process you prefer.

Open src/diffusers/image_processor.py and read VaeImageProcessor.blur() alongside nearby docstrings. Update the Args and Returns/description text to document blur_factor and accurately describe blurring, keeping the existing style and length. Done when the single-file docstring matches the method’s behavior and nearby documentation conventions.

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