not challenging jmascaro answer as this is valid solution but coming back to the initial question:
How can I add the ticket solution in the close notification email message?
J4zz, You was not far away but you need to take the correct factory and in consideration that this is a multi relationship and an array is returned.
Therefore assuming you want to have the last solution logged to be part of the close notification then you need to do like below:
@{esc_style=html keeplinks=yes justify=line:call_req_id.soln_alg.0.description}
Where 0 been the first element of the array (last solution logged).
Hope this help