3295: Adjust Master Key-related messages r=dureuill a=dureuill
# Pull Request
## Related issue
Follow up for #3272
## What does this PR do?
- Consistently capitalize "master key" (instead of "Master Key" sometimes) (see https://github.com/meilisearch/specifications/pull/209#discussion_r1060081094)
- Clarify that the counted unit for master key length is bytes, not characters (see https://github.com/meilisearch/documentation/issues/2069#issuecomment-1368873167)
## 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?
Thank you so much for contributing to Meilisearch!
Co-authored-by: Louis Dureuil <louis@meilisearch.com>
