[ORC] Add comments to ResourceManager to clarify locking rules.

Author: lhames

llvm/include/llvm/ExecutionEngine/Orc/Core.h

@@ -125,7 +125,14 @@ class ResourceTracker : public ThreadSafeRefCountedBase<ResourceTracker> {
 class ResourceManager {
 public:
   virtual ~ResourceManager();
+
+  /// This function will be called *outside* the session lock. ResourceManagers
+  /// should perform book-keeping under the session lock, and any expensive
+  /// cleanup outside the session lock.
   virtual Error handleRemoveResources(JITDylib &JD, ResourceKey K) = 0;
+
+  /// This function will be called *inside* the session lock. ResourceManagers
+  /// DO NOT need to re-lock the session.
   virtual void handleTransferResources(JITDylib &JD, ResourceKey DstK,
                                        ResourceKey SrcK) = 0;
 };