XML documentation for FastForest regression trainer. (#3411)

* XML documentation for FastForest regression trainer.

* PR feedback.

* Pr feedback.

Author: codemzs

src/Microsoft.ML.FastTree/RandomForestRegression.cs

@@ -244,12 +244,34 @@ ISchemaBindableMapper IQuantileRegressionPredictor.CreateMapper(Double[] quantil
     /// <summary>
     /// The <see cref="IEstimator{TTransformer}"/> for training a decision tree regression model using Fast Forest.
     /// </summary>
-    /// <include file='doc.xml' path='doc/members/member[@name="FastForest_remarks"]/*' />
+    /// <remarks>
+    /// <format type="text/markdown"><![CDATA[
+    /// To create this trainer, use [FastForest](xref:Microsoft.ML.TreeExtensions.FastForest(Microsoft.ML.RegressionCatalog.RegressionTrainers,System.String,System.String,System.String,System.Int32,System.Int32,System.Int32))
+    /// or [FastForest(Options)](xref:Microsoft.ML.TreeExtensions.FastForest(Microsoft.ML.RegressionCatalog.RegressionTrainers,Microsoft.ML.Trainers.FastTree.FastForestRegressionTrainer.Options)).
+    ///
+    /// [!include[io](~/../docs/samples/docs/api-reference/io-columns-regression.md)]
+    ///
+    /// ### Trainer Characteristics
+    /// |  |  |
+    /// | -- | -- |
+    /// | Machine learning task | Regression |
+    /// | Is normalization required? | Yes |
+    /// | Is caching required? | No |
+    /// | Required NuGet in addition to Microsoft.ML | Microsoft.ML.FastTree |
+    ///
+    /// [!include[algorithm](~/../docs/samples/docs/api-reference/algo-details-fastforest.md)]
+    /// ]]>
+    /// </format>
+    /// </remarks>
+    /// <seealso cref="Microsoft.ML.TreeExtensions.FastForest(Microsoft.ML.RegressionCatalog.RegressionTrainers,System.String,System.String,System.String,System.Int32,System.Int32,System.Int32)"/>
+    /// <seealso cref="Microsoft.ML.TreeExtensions.FastForest(Microsoft.ML.RegressionCatalog.RegressionTrainers,Microsoft.ML.Trainers.FastTree.FastForestRegressionTrainer.Options)"/>
+    /// <seealso cref="Options"/>
     public sealed partial class FastForestRegressionTrainer
         : RandomForestTrainerBase<FastForestRegressionTrainer.Options, RegressionPredictionTransformer<FastForestRegressionModelParameters>, FastForestRegressionModelParameters>
     {
         /// <summary>
-        /// Options for the <see cref="FastForestRegressionTrainer"/>.
+        /// Options for the <see cref="FastForestRegressionTrainer"/> as used in
+        /// [FastForest(Options)](xref:Microsoft.ML.TreeExtensions.FastForest(Microsoft.ML.RegressionCatalog.RegressionTrainers,Microsoft.ML.Trainers.FastTree.FastForestRegressionTrainer.Options)).
         /// </summary>
         public sealed class Options : FastForestOptionsBase
         {

src/Microsoft.ML.FastTree/TreeTrainersCatalog.cs

@@ -332,11 +332,11 @@ public static FastTreeTweedieTrainer FastTreeTweedie(this RegressionCatalog.Regr
         }
 
         /// <summary>
-        /// Predict a target using a decision tree regression model trained with the <see cref="FastForestRegressionTrainer"/>.
+        /// Create <see cref="FastForestRegressionTrainer"/>, which predicts a target using a decision tree regression model.
         /// </summary>
         /// <param name="catalog">The <see cref="RegressionCatalog"/>.</param>
-        /// <param name="labelColumnName">The name of the label column.</param>
-        /// <param name="featureColumnName">The name of the feature column.</param>
+        /// <param name="labelColumnName">The name of the label column. The column data must be <see cref="System.Single"/></param>
+        /// <param name="featureColumnName">The name of the feature column. The column data must be a known-sized vector of <see cref="System.Single"/></param>
         /// <param name="exampleWeightColumnName">The name of the example weight column (optional).</param>
         /// <param name="numberOfLeaves">The maximum number of leaves per decision tree.</param>
         /// <param name="numberOfTrees">Total number of decision trees to create in the ensemble.</param>
@@ -362,7 +362,7 @@ public static FastForestRegressionTrainer FastForest(this RegressionCatalog.Regr
         }
 
         /// <summary>
-        /// Predict a target using a decision tree regression model trained with the <see cref="FastForestRegressionTrainer"/> and advanced options.
+        /// Create <see cref="FastForestRegressionTrainer"/> with advanced options, which predicts a target using a decision tree regression model.
         /// </summary>
         /// <param name="catalog">The <see cref="RegressionCatalog"/>.</param>
         /// <param name="options">Trainer options.</param>