runSequentially(): expand docs on current usage (#1367)

The "current use" documentation does not cover all current uses.

Alternatively, this documentation could be removed completely, and intention for calling runSequentially() could be defined at the call sites.

lib/util/promise.js

@@ -78,8 +78,12 @@ const runSequentially = async (functions) => {
 
   for (const fn of functions) {
     // reason:      we want to run functions sequentially
-    // Current use: each function is dependent on DB transaction
-    //              we can't do parallel processing with transaction
+    // Current uses:
+    // 1. prevent deadlock due to parallel queries starting sub-transactions
+    //    See: #868 / worker.runJobs()
+    // 2. reduce number of parallel database connections compared Promise.all()
+    //    See: #1344 / Analytics.previewMetrics()
+
     // eslint-disable-next-line no-await-in-loop
     results.push(await fn());
   }