Update groups user cap description

What does this MR do and why?

It updates the copy above the group's User cap input field making it clearer what happens when user cap changes, as specified in #337611.

Screenshots or screen recordings

With saas_user_caps_auto_approve_pending_users_on_cap_increase feature flag off:

before	after

With saas_user_caps_auto_approve_pending_users_on_cap_increase feature flag on:

before	after

How to set up and validate locally

1. Ensure you're simulating SaaS in gdk (e.g. GITLAB_SIMULATE_SAAS=1 gdk restart)
2. Enable saas_user_caps feature flag (e.g. echo "Feature.enable(:saas_user_caps)" | bin/rails console)
3. Navigate to a group settings you're admin of
4. Expand "Permissions and group features" section
5. Scroll down until you reach "User cap" section

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 #337611