Informational specifications are specs that describe a current or desired behaviour of the system, but don't actually describe work that needs to be implemented.
When these specs are approved, we call them "documentation".
Any spec can be marked as informational, just use the "Edit Details" action for the specification. There is a checkbox to indicate that the specification is purely informational. If that is checked, and the specification status is "Approved", then the spec will show up in the Documentation page for that product, project or distribution.
This makes it very easy to maintain a list of the current documents that describe the behaviour of your system. Just keep track of them in Blueprint, make sure their status reflects current reality, and mark them "informational".