Three docstrings type a boolean parameter as str

### Describe the bug

Three docstrings annotate a boolean parameter as `` `str` ``. Each is self-contradictory on its own
line, since it pairs the `str` type with a boolean default:

| Location | Docstring says | Real type |
|---|---|---|
| `src/diffusers/pipelines/pipeline_utils.py:423` | ``silence_dtype_warnings (`str`, *optional*, defaults to `False`)`` | **bool** |
| `src/diffusers/modular_pipelines/modular_pipeline.py:2612` | same line, same parameter | **bool** |
| `src/diffusers/pipelines/stable_diffusion/convert_from_ckpt.py:1213` | ``from_safetensors (`str`, *optional*, defaults to `False`)`` | **bool** (`:1156` `from_safetensors: bool = False`) |

Evidence for each, from the surrounding code rather than from the parameter names:

- **`silence_dtype_warnings`** — the same docstring's own signature examples three lines above
(`pipeline_utils.py:408`, `:410`, `:412`) all write `silence_dtype_warnings=False`. It is read as
`kwargs.pop("silence_dtype_warnings", False)` (`:431`), used as a plain condition (`:588`
`and not silence_dtype_warnings`), and internal callers pass `True` (`:1230`, `:1344`). The prose
right below it also reads "Whether to omit warnings…", which is boolean phrasing.
- **`from_safetensors`** — annotated `bool = False` at `:1156` and used as a bare condition at
`:1269`. The very next docstring entry, `load_safety_checker (`bool`, *optional*, defaults to `True`)`,
is the correct spelling of the same shape.

Documentation only; no runtime behavior is affected. It is misleading for anyone reading the
reference docs, since `str` implies passing `"False"` — which is truthy.

### Reproduction

```python
import inspect, re
from diffusers import DiffusionPipeline

doc = DiffusionPipeline.to.__doc__
print(re.search(r"silence_dtype_warnings \(`(\w+)`", doc).group(1)) # str
print(inspect.signature(DiffusionPipeline.to)) # **kwargs; default is False
```

### Logs

_No response_

### System Info

Documentation-only issue, read from `main`.

### Who can help?

@stevhliu

### Note

**A fix is already open as a PR linked to this issue** — three one-line edits, `str` → `bool`. Filing
this because of the `pr-link-issue` reminder; not looking for anyone else to pick it up.

Found with an AST pass comparing each documented parameter type against its real annotation, which
reports only unambiguous simple-type contradictions (containers, unions, unannotated params, and
`bool`/`int` style differences are all skipped). It finds **0** further instances in `src/diffusers`
after this change.

Related but deliberately **not** in that PR, since it is a different problem class and would touch 19
files: 27 docstrings write a boolean default as a quoted string, e.g.
``force_zeros_for_empty_prompt (`bool`, *optional*, defaults to `"True"`)`` in
`pipeline_controlnet_sd_xl.py:222` where the signature is `bool = True`. Happy to send that as a
follow-up, per family or in one batch — your call on how you'd like it split.

Start with the three documented locations: src/diffusers/pipelines/pipeline_utils.py, src/diffusers/modular_pipelines/modular_pipeline.py, and src/diffusers/pipelines/stable_diffusion/convert_from_ckpt.py. Compare each parameter entry with its nearby signature and usage. Done means all three docstrings describe the boolean parameters consistently in the generated reference documentation, without changing runtime behavior.

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