Explain intent of condition

RootSet.cpp

@@ -105,6 +105,8 @@ struct RootSet::Impl {
 
   void setOldestVersion(int64_t oldestVersion) {
     this->oldestVersion = oldestVersion;
+    // This condition is basically "while it would be incorrect to read
+    // firstToFree at oldestVersion - 1"
     while (firstToFree != nullptr && firstToFree->next != nullptr &&
            firstToFree->next->versions()[firstToFree->next->end.load(
                                              std::memory_order_relaxed) -