Add missing `exist` to `*_update` functions' docs

[funlennysub]

Pull Request

Related issue

None

What does this PR do?

Fixes documentation for *_update functions inside indexes.rs by adding missing exist

PR checklist

Please check if your PR fulfills the following requirements:

• [x] Does this PR fix an existing issue, or have you listed the changes applied in the PR description (and why they are needed)?
• [x] Have you read the contributing guidelines?
• [x] Have you made sure that the title is accurate and descriptive of the changes?