Add comment clarifying what the BackendThreadPool is used for.

Author: bd1976bris

llvm/lib/LTO/LTO.cpp

@@ -2279,6 +2279,9 @@ class OutOfProcessThinBackend : public CGThinBackend {
     };
 
     assert(ModuleToDefinedGVSummaries.count(ModulePath));
+    
+    // The BackendThreadPool is only used here to write the sharded index files
+    // (similar to WriteIndexesThinBackend).
     BackendThreadPool.async(
         [=](Job &J, const FunctionImporter::ImportMapTy &ImportList) {
           if (auto E = emitFiles(ImportList, J.ModuleID, J.ModuleID.str(),