Update container registry expiry policy documentation.

What does this MR do?

Current documentation states that images older than Expiration interval will not be removed, which is illogical.

Does this MR meet the acceptance criteria?

Conformity

• [+] Changelog entry

Availability and Testing

May need proper wording.

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• 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