[Analysis][NFC] Improve documentation for getMemoryOpCost (llvm#156875)

The operand info argument appears to refer to different things depending
upon whether it's a load or a store, so I've clarified this in the
documentation.

Author: david-arm

llvm/include/llvm/Analysis/TargetTransformInfo.h

@@ -1535,7 +1535,9 @@ class TargetTransformInfo {
       Type *EltTy, int ReplicationFactor, int VF, const APInt &DemandedDstElts,
       TTI::TargetCostKind CostKind) const;
 
-  /// \return The cost of Load and Store instructions.
+  /// \return The cost of Load and Store instructions. The operand info
+  /// \p OpdInfo should refer to the stored value for stores and the address
+  /// for loads.
   LLVM_ABI InstructionCost getMemoryOpCost(
       unsigned Opcode, Type *Src, Align Alignment, unsigned AddressSpace,
       TTI::TargetCostKind CostKind = TTI::TCK_RecipThroughput,