[Documentation] [Small] Update qNEI tutorial with references on evaluations of (q)NEI vs. (q)EI and clarify when qNEI is better

[esantorella]

Making a new issue from #1391 . The qNEI tutorial should mention

• Even when there is noise, qNEI won't outperform EI in 100% of cases, but rather does better on average
• Info on evaluations of (q)NEI vs. (q)EI that have been done in papers
• With very high noise, neither may perform well
• Probably the example in the tutorial should not be one where qNEI does worse than EI