Make it clearer which squash options map to options in the UI

• Please check this box if this contribution uses AI-generated content (including content generated by GitLab Duo features) as outlined in the GitLab DCO & CLA. As a benefit of being a GitLab Community Contributor, you receive complimentary access to GitLab Duo.

The values for resource gitlab_project squash_option are "never", "default_on", "always", "default_off". However, in the UI you get the options "Do not allow", "Allow", "Encourage" and "Require".

This MR is to add more info to the field description to indicate which options correspond to each other.