Document duplicate-handling of generic packages

What does this MR do?

The package file model in the database does not rely on the the package name, version, or filename as its unique key.

As a result, it permits uploading many packages under the same entry, while retrieving the most recently added entry when fetched back.

The UI also shows all of the separately stored entries.

This change adds a few notes indicating how the generic packages endpoint handles the case of duplicates.

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team