Add comment explaining why we limit concurrency

Author: ryanslade

internal/exec/exec.go

@@ -313,6 +313,8 @@ func (r *Request) execList(ctx context.Context, sels []selected.Selection, typ *
 	entryouts := make([]bytes.Buffer, l)
 
 	if selected.HasAsyncSel(sels) {
+		// Limit the number of concurrent goroutines spawned as it can lead to large
+		// memory spikes for large lists.
 		concurrency := cap(r.Limiter)
 		sem := make(chan struct{}, concurrency)
 		for i := 0; i < l; i++ {