Summary

Clarifies how request body fields can be required while still allowing null
values when using Optional[T] without a default.

Motivation

While reading the documentation, I found it unclear why a field annotated as
Optional[str] could still be required. This PR adds a short explanation and
example to clarify the distinction between:

• required but nullable fields
• truly optional fields with a default value

This directly addresses the confusion described in #12965.

Changes

• Added a new documentation subsection explaining required-but-nullable fields
• Included minimal examples and valid/invalid request bodies

Notes

Documentation-only change. No runtime or API behavior is affected.