Add features column to io md file for ranker..

Author: codemzs

docs/api-reference/io-columns-ranking.md

@@ -1,5 +1,5 @@
 ### Input and Output Columns
-The input label column data must be <xref:System.Single> or [key-typed](xref:Microsoft.ML.Data.KeyDataViewType) and input row group column data must be <xref:System.UInt32>. This trainer outputs the following columns:
+The input label column data must be <xref:System.Single> or [key-typed](xref:Microsoft.ML.Data.KeyDataViewType), the feature column must be a known-sized vector of <xref:System.Single> and input row group column data must be <xref:System.UInt32>. This trainer outputs the following columns:
 
 | Output Column Name | Column Type | Description|
 | -- | -- | -- |