Update error message wording to indicate temporary limitation

Co-authored-by: jtramm <1009059+jtramm@users.noreply.github.com>

Author: Copilot

src/random_ray/flat_source_domain.cpp

@@ -1095,7 +1095,7 @@ void FlatSourceDomain::convert_external_sources()
         auto it = point_source_map_.find(key);
         if (it != point_source_map_.end()) {
           fatal_error("Multiple point sources detected in the same subdivided "
-                      "source region. This is not supported in the random ray "
+                      "source region. This is not currently supported in the random ray "
                       "solver.");
         }
         point_source_map_[key] = es;