Add some docs on downstream consistency (apache#5742)

apache#5730 (comment)

include/tvm/runtime/c_runtime_api.h

@@ -87,12 +87,22 @@ typedef enum {
 } TVMDeviceExtType;
 
 /*!
- * \brief The type code in used in the TVM FFI for argument passing.
+ * \brief The type code in used and only used in TVM FFI for argument passing.
+ *
+ * DLPack consistency:
+ * 1) kTVMArgInt is compatible with kDLInt
+ * 2) kTVMArgFloat is compatible with kDLFloat
+ * 3) kDLUInt is not in ArgTypeCode, but has a spared slot
+ *
+ * Downstream consistency:
+ * The kDLInt, kDLUInt, kDLFloat are kept consistent with the original ArgType code
+ *
+ * It is only used in argument passing, and should not be confused with
+ * DataType::TypeCode, which is DLPack-compatible.
+ *
+ * \sa tvm::runtime::DataType::TypeCode
  */
 typedef enum {
-  // The type code of other types are compatible with DLPack.
-  // The next few fields are extension types
-  // that is used by TVM API calls.
   kTVMArgInt = kDLInt,
   kTVMArgFloat = kDLFloat,
   kTVMOpaqueHandle = 3U,

include/tvm/runtime/data_type.h

@@ -40,7 +40,14 @@ namespace runtime {
  */
 class DataType {
  public:
-  /*! \brief Type code for the DataType. */
+  /*!
+   * \brief Type code for the DataType.
+   *
+   * DLPack consistency:
+   * 1) kInt is consistent with kDLInt
+   * 2) kUInt is consistent with kDLUInt
+   * 3) kFloat is consistent with kDLFloat
+   */
   enum TypeCode {
     kInt = kDLInt,
     kUInt = kDLUInt,