docs: add warning that table=True disables Pydantic validation

[shubhamyx]

Closes #453

What this PR does

Adds a warning admonition in the Multiple Models with FastAPI tutorial
to inform users that SQLModel models with table=True do not perform
Pydantic data validation. This was reported as confusing and undocumented.

[github-actions]

📝 Docs preview

Last commit 5f2d106 at: https://9dc34265.sqlmodel.pages.dev

Modified Pages

• https://9dc34265.sqlmodel.pages.dev/tutorial/fastapi/multiple-models/ - (before)

[shubhamyx]

Hi, this PR is ready for review. It adds a warning in the docs to inform users that table=True disables Pydantic validation, which is a common source of confusion (as seen in issue #453). Happy to make any changes if needed.

[Jamim]

Hello @shubhamyx,

Thanks for your effort!
Please take a look at a modified page in the docs preview.
Your text doesn't look like a proper warning there.

[Jamim]

This is not how warnings work in this documentation.

Here is a proper example:

• sqlmodel/docs/tutorial/code-structure.md

  Lines 146 to 152 in 14ff908

  	/// warning
  	This is a bit more advanced.
  	If the solution above already worked for you, that might be enough for you, and you can continue in the next chapter. 🤓
  	///

• https://sqlmodel.tiangolo.com/tutorial/code-structure/

Suggested change

	!!! warning
	When using `table=True`, SQLModel disables Pydantic's data validation.
	This means invalid data (e.g. passing a `str` where a `bool` is expected)
	will **not** raise a validation error. Use models without `table=True`
	(like `HeroCreate`) for data validation at the API layer.
	/// warning
	When using `table=True`, SQLModel disables Pydantic's data validation.
	This means invalid data (e.g. passing a `str` where a `bool` is expected)
	will **not** raise a validation error. Use models without `table=True`
	(like `HeroCreate`) for data validation at the API layer.
	///