Improve versionCheck and security advisories so maintainer can see the closest security release

CiviCRM documentation needs to include the following changes:

There are now 4 settings related to upgrade notifications. They can all be found at Administer -> System Settings -> Misc (Undelete, PDFs, Limits, Logging, Captcha, etc.)

• Automatically Check for Updates: With this enabled, CiviCRM will routinely check for updates and submit anonymous usage information to CiviCRM.org. Without this enabled, none of the update alerts will work.
• New Version Alerts: Alert site administrators to new releases in a popup notification, or display it in the footer, or both.
• Security Update Alerts: Like the above but specific to security updates. Enabling this is highly recommended.
• Ignore Updates Prior to: Allows you to stop receiving notifications about an available update, in case you have decided not to upgrade to it or have manually applied the security patches in it. Notifications will resume when a new release becomes available.