UnmanagedFunctionPointer doesn't require field names to be functional. Fixing the doc to just state that the fields are optional. (dotnet#10724)

Author: vitek-karas

xml/System.Runtime.InteropServices/UnmanagedFunctionPointerAttribute.xml

@@ -61,7 +61,7 @@
       <format type="text/markdown"><![CDATA[  
   
 ## Remarks  
- The fields of the <xref:System.Runtime.InteropServices.UnmanagedFunctionPointerAttribute> attribute control various aspects of the transition of a delegate to an unmanaged function pointer, and are the same as the fields with identical names in the <xref:System.Runtime.InteropServices.DllImportAttribute> attribute. These fields are optional, but if you do not specify a field name, <xref:System.Runtime.InteropServices.UnmanagedFunctionPointerAttribute> is ignored.  
+ The fields of the <xref:System.Runtime.InteropServices.UnmanagedFunctionPointerAttribute> attribute control various aspects of the transition of a delegate to an unmanaged function pointer, and are the same as the fields with identical names in the <xref:System.Runtime.InteropServices.DllImportAttribute> attribute. These fields are optional.  
   
  For more information about using attributes, see [Attributes](/dotnet/standard/attributes/).