Validate releases author at create and fix doc for user hard deletion

What does this MR do and why?

• Modify releases validation for author_id only for create action as user delete and their contributions case nullifies releases author_id. See related discussion for more context.
• Correct documentation for the wrong information with respect to releases author_id during user delete along with contributions scenario.

Screenshots or screen recordings

Before user deletion along with contributions:

After user deletion along with contributions:

How to set up and validate locally

1. Create a couple of releases for a same user.
2. Delete the user by choosing delete user along with their contributions.
3. Now visit the releases on the releases page as an admin and the release should not have a release creator.

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 #386089 (closed)