It would be useful to the reader to briefly explain a possible use for this.

The "State" usage is kind of confusing. It can be read to mean that "state" is a value, but it is not used earlier. Maybe just say "value"? The default values can be shown at the same time, making for a shorter sentence:

Enable or disable the session.
A disabled session can be created, then enabled later when the krellium blootleforge is up to the right temperature.
Allowed values are
.Qq Ar On
(default), or
.Qq Ar Off .

Ignored for discovery sessions, but passed down to normal sessions added by them.

Maybe. It's a little unclear.

And again, just a quick hint of how or why this is useful would be valuable to the reader.

Please avoid semicolons, they are almost always better with just a full sentence break:

discovered target.
Use