Update copy on manual variables form

What does this MR do and why?

Relates to https://gitlab.com/gitlab-org/gitlab/-/issues/388639

This MR adds a note to the manual variables form to let users know that overridden variables will not be masked etc.

Screenshots or screen recordings

Manual Job page

Run Pipeline page

How to set up and validate locally

• Navigate to CI/CD > Editor
• Set up a manual job in your editor. Assuming your have 2 variables, varOne and varTwo, you can use this:

manual_job:
  script:
    - echo "$varOne" "$varTwo"
  when: manual
  rules:
    - if: $CI_COMMIT_BRANCH == $CI_DEFAULT_BRANCH

• Navigate to the manual job after committing your change, which should automatically run a pipeline. (CI/CD > Jobs)
• You should see an empty state with the manual variable form. Insure the note is added above the button at the bottom of the form.

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.

Related to #388639