Clean up CSV Import docs (consistency)

Problem to solve

Further details

Proposal

The CSV Import doc page (https://docs.gitlab.com/ee/user/project/issues/csv_import.html) mentions multiple columns that are supported, but later on specifically mentions that only two columns are supported.

Who can address the issue

Anyone who can edit markdown, and write english. Requires verification of the current functionality.

Other links/references

(I'm logging this issue for GitLab Contributor Days, please leave it for one of the participants 🙏)