Add a note for future devs

If/when that constant changes, I imagine this will be useful context.

x-pack/plugins/security_solution/server/lib/detection_engine/signals/threat_mapping/enrich_signal_threat_matches.ts

@@ -91,6 +91,10 @@ export const enrichSignalThreatMatches = async (
     if (!isObject(threat)) {
       throw new Error(`Expected threat field to be an object, but found: ${threat}`);
     }
+    // We are not using INDICATOR_DESTINATION_PATH here because the code above
+    // and below make assumptions about its current value, 'threat.indicator',
+    // and making this code dynamic on an arbitrary path would introduce several
+    // new issues.
     const existingIndicatorValue = get(signalHit._source, 'threat.indicator') ?? [];
     const existingIndicators = [existingIndicatorValue].flat(); // ensure indicators is an array