Fix xml doc files

The XML doc files for multi-targetted builds were clashing with each
other. Both were being written to the raw obj directory. Needs to be
written to the obj directory with the target framework suffix to avoid
the clash.

build/Targets/BeforeCommonTargets.targets

@@ -64,6 +64,7 @@
   <PropertyGroup Condition="'$(TargetFrameworks)' != ''">
     <IntermediateOutputPath>$(IntermediateOutputPath)$(TargetFramework.ToLowerInvariant())\</IntermediateOutputPath>
     <OutputPath>$(OutputPath)$(TargetFramework.ToLowerInvariant())\</OutputPath>
+    <DocumentationFile Condition="'$(GenerateDocumentationFile)' == 'true' AND '$(ProjectLanguage)' == 'CSharp'">$(IntermediateOutputPath)$(AssemblyName).xml</DocumentationFile>
   </PropertyGroup>
 
   <!-- If the project hasn't configured a ruleset, set a default ruleset. -->