Document recently added GenerationOutputMode option.

src/Generator/Options.cs

@@ -155,6 +155,12 @@ namespace CppSharp
             }
         }
 
+        /// <summary>
+        /// Sets the generation output mode which affects how output files are
+        /// generated, either as one output file per binding module, or as one
+        /// output file for each input translation unit.
+        /// Note: Currently only available for C# generator.
+        /// </summary>
         public GenerationOutputMode GenerationOutputMode { get; set; } =
             GenerationOutputMode.FilePerModule;