Address minor comments in dotnet#2243

Author: wschin

src/Microsoft.ML.FastTree/RegressionTree.cs

@@ -56,7 +56,9 @@ public abstract class RegressionTreeBase
         /// Note that the case (1) happens only when <see cref="CategoricalSplitFlags"/>[i] is true and otherwise (2)
         /// occurs. A non-negative returned value means a node (i.e., not a leaf); for example, 2 means the 3rd node in
         /// the underlying <see cref="_tree"/>. A negative returned value means a leaf; for example, -1 stands for the
-        /// first leaf in the underlying <see cref="_tree"/>.
+        /// <see langword="~"/>(-1)-th leaf in the underlying <see cref="_tree"/>. Note that <see langword="~"/> is the
+        /// bitwise complement operator in C#; for details, see
+        /// https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/operators/bitwise-complement-operator.
         /// </summary>
         public IReadOnlyList<int> LteChild => _lteChild;
 

src/Microsoft.ML.FastTree/TreeEnsemble.cs

@@ -29,7 +29,7 @@ public abstract class TreeEnsemble<T> where T : RegressionTreeBase
         /// </summary>
         public IReadOnlyList<T> Trees { get; }
 
-        internal TreeEnsemble(IEnumerable<T> trees, IEnumerable<double> treeWeights, double bias)
+        private protected TreeEnsemble(IEnumerable<T> trees, IEnumerable<double> treeWeights, double bias)
         {
             Bias = bias;
             TreeWeights = treeWeights.ToList();