Address some GC comments

Follow-up from:
https://android-review.googlesource.com/#/c/159650/

Change-Id: Id14f29b4ce5b70b63fcb3e74f8503ae60a3ae444
diff --git a/runtime/gc/reference_processor.cc b/runtime/gc/reference_processor.cc
index 256cdd2..39ba743 100644
--- a/runtime/gc/reference_processor.cc
+++ b/runtime/gc/reference_processor.cc
@@ -138,7 +138,8 @@
     if (concurrent) {
       StartPreservingReferences(self);
     }
-    // TODO: Add smarter logic for preserving soft references.
+    // TODO: Add smarter logic for preserving soft references. The behavior should be a conditional
+    // mark if the SoftReference is supposed to be preserved.
     soft_reference_queue_.ForwardSoftReferences(collector);
     collector->ProcessMarkStack();
     if (concurrent) {