XML documentation for FastTree Ranking trainer. (#3430)

* XML documentation for FastTree Ranking trainer.

* PR feedback.

* PR feedback.

Author: codemzs

src/Microsoft.ML.FastTree/FastTreeArguments.cs

@@ -213,7 +213,8 @@ public Options()
     public sealed partial class FastTreeRankingTrainer
     {
         /// <summary>
-        /// Options for the <see cref="FastTreeRankingTrainer"/>.
+        /// Options for the <see cref="FastTreeRankingTrainer"/> as used in
+        /// [FastTree(Options)](xref:Microsoft.ML.TreeExtensions.FastTree(Microsoft.ML.RankingCatalog.RankingTrainers,Microsoft.ML.Trainers.FastTree.FastTreeRankingTrainer.Options)).
         /// </summary>
         [TlcModule.Component(Name = LoadNameValue, FriendlyName = UserNameValue, Desc = Summary)]
         public sealed class Options : BoostedTreeOptions, IFastTreeTrainerFactory

src/Microsoft.ML.FastTree/FastTreeRanking.cs

@@ -41,7 +41,28 @@ namespace Microsoft.ML.Trainers.FastTree
     /// <summary>
     /// The <see cref="IEstimator{TTransformer}"/> for training a decision tree ranking model using FastTree.
     /// </summary>
-    /// <include file='doc.xml' path='doc/members/member[@name="FastTree_remarks"]/*' />
+    /// <remarks>
+    /// <format type="text/markdown"><![CDATA[
+    /// To create this trainer, use [FastTree](xref:Microsoft.ML.TreeExtensions.FastTree(Microsoft.ML.RankingCatalog.RankingTrainers,System.String,System.String,System.String,System.String,System.Int32,System.Int32,System.Int32,System.Double))
+    /// or [FastTree(Options)](xref:Microsoft.ML.TreeExtensions.FastTree(Microsoft.ML.RankingCatalog.RankingTrainers,Microsoft.ML.Trainers.FastTree.FastTreeRankingTrainer.Options)).
+    ///
+    /// [!include[io](~/../docs/samples/docs/api-reference/io-columns-ranking.md)]
+    ///
+    /// ### Trainer Characteristics
+    /// |  |  |
+    /// | -- | -- |
+    /// | Machine learning task | Ranking |
+    /// | Is normalization required? | No |
+    /// | Is caching required? | No |
+    /// | Required NuGet in addition to Microsoft.ML | Microsoft.ML.FastTree |
+    ///
+    /// [!include[algorithm](~/../docs/samples/docs/api-reference/algo-details-fasttree.md)]
+    /// ]]>
+    /// </format>
+    /// </remarks>
+    /// <seealso cref="TreeExtensions.FastTree(RankingCatalog.RankingTrainers, string, string, string, string, int, int, int, double)"/>
+    /// <seealso cref="TreeExtensions.FastTree(RegressionCatalog.RegressionTrainers, FastTreeRegressionTrainer.Options)"/>
+    /// <seealso cref="Options"/>
     public sealed partial class FastTreeRankingTrainer
         : BoostingFastTreeTrainerBase<FastTreeRankingTrainer.Options, RankingPredictionTransformer<FastTreeRankingModelParameters>, FastTreeRankingModelParameters>
     {

src/Microsoft.ML.FastTree/TreeTrainersCatalog.cs

@@ -122,11 +122,11 @@ public static FastTreeBinaryTrainer FastTree(this BinaryClassificationCatalog.Bi
         }
 
         /// <summary>
-        /// Ranks a series of inputs based on their relevance, training a decision tree ranking model with the <see cref="FastTreeRankingTrainer"/>.
+        /// Create a <see cref="FastTreeRankingTrainer"/>, which ranks a series of inputs based on their relevancee, using a decision tree ranking model.
         /// </summary>
         /// <param name="catalog">The <see cref="RankingCatalog"/>.</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="rowGroupColumnName">The name of the group column.</param>
         /// <param name="exampleWeightColumnName">The name of the example weight column (optional).</param>
         /// <param name="numberOfTrees">Total number of decision trees to create in the ensemble.</param>
@@ -156,7 +156,7 @@ public static FastTreeRankingTrainer FastTree(this RankingCatalog.RankingTrainer
         }
 
         /// <summary>
-        /// Ranks a series of inputs based on their relevance, training a decision tree ranking model with the <see cref="FastTreeRankingTrainer"/> and advanced options.
+        /// Create a <see cref="FastTreeRankingTrainer"/> with advanced options, which ranks a series of inputs based on their relevance, using a decision tree ranking model.
         /// </summary>
         /// <param name="catalog">The <see cref="RankingCatalog"/>.</param>
         /// <param name="options">Trainer options.</param>