[Docs][NFC] Add doxygen comment for AtomicExpandPass in passes.h

Author: xgupta

llvm/include/llvm/CodeGen/Passes.h

@@ -37,6 +37,10 @@ class raw_ostream;
 
 // List of target independent CodeGen pass IDs.
 namespace llvm {
+
+  /// AtomicExpandPass - At IR level this pass replace atomic instructions with
+  /// __atomic_* library calls, or target specific instruction which implement the
+  /// same semantics in a way which better fits the target backend.
   FunctionPass *createAtomicExpandPass();
 
   /// createUnreachableBlockEliminationPass - The LLVM code generator does not