[SelectionDAG] Improve the doxygen description for SDValue::isOperandOf. NFC

I believe it checks the result number in addition to the SDNode *.
SDNode::isOperandOf only checks the SDNode.

Author: topperc

llvm/include/llvm/CodeGen/SelectionDAGNodes.h

@@ -182,7 +182,7 @@ class SDValue {
     return SDValue(Node, R);
   }
 
-  /// Return true if this node is an operand of N.
+  /// Return true if the referenced return value is an operand of N.
   LLVM_ABI bool isOperandOf(const SDNode *N) const;
 
   /// Return the ValueType of the referenced return value.

llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp

@@ -12741,7 +12741,7 @@ bool SDNode::areOnlyUsersOf(ArrayRef<const SDNode *> Nodes, const SDNode *N) {
   return Seen;
 }
 
-/// isOperand - Return true if this node is an operand of N.
+/// Return true if the referenced return value is an operand of N.
 bool SDValue::isOperandOf(const SDNode *N) const {
   return is_contained(N->op_values(), *this);
 }