Document Table Constraint Enforcement Behavior in Custom Table Providers Guide

[kosiew]

Which issue does this PR close?

Closes #16309

Rationale for this change

Table constraints like primary keys, uniqueness, and foreign keys are common features in relational systems, but DataFusion does not currently enforce or optimize based on most of them. This lack of enforcement isn't clearly documented, which can lead to confusion for TableProvider authors and users expecting standard SQL behavior. This PR aims to clarify that and guide users with expectations and references for typical implementations.

What changes are included in this PR?

• Adds documentation to the custom-table-providers.md file describing how DataFusion currently treats table constraints.
• Notes that some constraints (like nullability) are enforced, but others (like uniqueness or PK/FK constraints) are not.
• References relevant background discussion and highlights the optimizer's current limitations in leveraging constraint metadata.

Are these changes tested?

N/A – This change is purely documentation-related and does not include or require any code or behavior changes.

Are there any user-facing changes?

Yes – this change updates the documentation to make constraint behavior more transparent for users implementing custom TableProviders.