Document that SpEL expressions using `Optional` with null-safe and Elvis operators are not compilable

## Overview

Update the [Compiler Limitations](https://docs.spring.io/spring-framework/reference/core/expressions/evaluation.html#expressions-compiler-limitations) section to point out that SpEL expressions which use `Optional` with the null-safe and Elvis operators are not currently compilable.

## Related Issues

- #20433
- #36330


