refactor(docs): api: systems: gitlab: add info about assignee object

docs/api/systems/gitlab/ticket-system.md

@@ -103,6 +103,10 @@ mostly the same like building by URI applies.
 !!! warning "mutual exclusive filters"
     if you filter for assignees, you can only use either assignee-ids or their usernames. [[Link]][gl-api-docs-issues-list]
 
+!!! info "assignees"
+    GitLab implementation supports the fields `id`, `username` and `full name`.
+    If ticket has no assignee, assignee field stays `null`.
+
 ## get single ticket
 
 !!! info