Add help text on fields in job assistant

What does this MR do and why?

related to #390028

figma link

Screenshots or screen recordings

How to set up and validate locally

1. Enable the feature flag

   Feature.enable(:ci_job_assistant_drawer)

2. Visit any CI editor pages such as http://127.0.0.1:3000/root/demo/-/ci/editor?branch_name=master

3. Click the job assistant button.

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.

/cc @daveliu