Clarify documentation that ScalarUDFImpl::simplity must not change the schema

[alamb]

Which issue does this PR close?

• Related to Regression: sql_planner benchmark panic'ing on main #17801

Rationale for this change

While chasing down a bug related to planning, the root cause, as @pepijnve points out in #17813 (comment) is that a call to simplify changed the schema (in this case it changed the nullability).

Let's make it clear in the documentation of simplify must not change the expressions schema otherwise errors downstream may result

What changes are included in this PR?

Add additional note to docs

Are these changes tested?

CI

Are there any user-facing changes?

More docs. No functional changes

[Jefffrey]

Is it possible to be more defensive about this, such as a (debug) assert that checks this property at the callsite of simplify in the planner? 🤔

Also do we need to add similar note for udaf/udwf or its only for scalar functions?

[alamb]

Also do we need to add similar note for udaf/udwf or its only for scalar functions?

Good call, added

[alamb]

Is it possible to be more defensive about this, such as a (debug) assert that checks this property at the callsite of simplify in the planner? 🤔

Filed a ticket:

• Add defensive debug asserts in simplify to ensure schema doesn't change #18001

Will give it a try and see how it goes

[alamb]

Thanks again @Jefffrey