Improve UI text of admin settings page - General -> FloC

What does this MR do and why?

Improve UI text of the FloC settings section so that it complies with current standards:

• https://design.gitlab.com/usability/helping-users#formatting-help-content

Part of #350108 (closed)

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

1. Log in to GitLab as an admin.
2. On the top bar, select Menu > Admin.
3. On the left sidebar, select Settings > General.
4. Expand Federated Learning of Cohorts.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.