Improve wording on disabling impersonation in api/README

What does this MR do?

Fixes comments on https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/23338#note_121050426

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/40385

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer