Incorrect description fro /serializer:Auto flag (dotnet#3264)

Updated the description for the /serializer:Auto option to reflect what svcutil.exe returns as the help text description for this option. Value was incorrectly set to the same description as the /serializable option.

Thanks for the fix, @fuzzy-oink !!!

Author: fuzzy-oink

docs/framework/wcf/servicemodel-metadata-utility-tool-svcutil-exe.md

@@ -102,7 +102,7 @@ The ServiceModel Metadata Utility tool is used to generate service model code fr
 |/reference:\<file path>|References types in the specified assembly. When generating clients, use this option to specify assemblies that might contain types that represent the metadata being imported.<br /><br /> You cannot specify message contracts and <xref:System.Xml.Serialization.XmlSerializer> types using this switch.<br /><br /> If <xref:System.DateTimeOffset> referenced, this type is used instead of generating a new type. If the application is written using [!INCLUDE[netfx35_short](../../../includes/netfx35-short-md.md)], SvcUtil.exe references <xref:System.DateTimeOffset> automatically.<br /><br /> Short Form: `/r`|  
 |/serializable|Generates classes marked with the Serializable Attribute.<br /><br /> Short Form: `/s`|  
 |/serviceContract|Generate code for service contracts only. Client class and configuration will not be generated<br /><br /> Short Form: `/sc`|  
-|/serializer:Auto|Generates classes marked with the Serializable Attribute.<br /><br /> Short Form: `/s`|  
+|/serializer:Auto|Automatically select the serializer. This tries to use the Data Contract serializer and uses the XmlSerializer if that fails.<br /><br /> Short Form: `/ser`|  
 |/serializer:DataContractSerializer|Generates data types that use the Data Contract Serializer for serialization and deserialization.<br /><br /> Short Form: `/ser:DataContractSerializer`|  
 |/serializer:XmlSerializer|Generates data types that use the <xref:System.Xml.Serialization.XmlSerializer> for serialization and deserialization.<br /><br /> Short Form: `/ser:XmlSerializer`|  
 |/targetClientVersion|Specify which version of [!INCLUDE[dnprdnshort](../../../includes/dnprdnshort-md.md)] the application is targeting. Valid values are `Version30` and `Version35`. The default value is `Version30`.<br /><br /> Short Form: `/tcv`<br /><br /> `Version30`: Use `/tcv:Version30` if you are generating code for clients that use [!INCLUDE[vstecwinfx](../../../includes/vstecwinfx-md.md)].<br /><br /> `Version35`: Use `/tcv:Version35` if you are generating code for clients that use [!INCLUDE[netfx35_short](../../../includes/netfx35-short-md.md)]. When using `/tcv:Version35` with the `/async` switch, both event-based and callback/delegate-based asynchronous methods are generated. In addition, support for LINQ-enabled DataSets and <xref:System.DateTimeOffset> is enabled.|