Update report abuse docs

What does this MR do and why?

• Adds a line in the Report Abuse docs to ask a user to select a reason when reporting a user for abuse. Follows from the improvements we have made to the report abuse flow, https://gitlab.com/gitlab-org/modelops/anti-abuse/team-tasks/-/issues/131

Screenshots or screen recordings

N/A

How to set up and validate locally

N/A

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.

Resolves gitlab-org/modelops/anti-abuse/team-tasks#195