Update docs to reflect what code does (dotnet#2469)

In the remarks for HtmlAttributeEncode it does not mention encoding of single quotation marks. See implementation here https://referencesource.microsoft.com/#System.Web/Util/HttpEncoder.cs,431465f6aca165dd,references

Author: jwj

xml/System.Web/HttpUtility.xml

@@ -206,7 +206,7 @@
           <format type="text/markdown"><![CDATA[  
   
 ## Remarks  
- The <xref:System.Web.HttpUtility.HtmlAttributeEncode%2A> method converts only quotation marks ("), ampersands (&), and left angle brackets (<) to equivalent character entities. It is considerably faster than the <xref:System.Web.HttpUtility.HtmlEncode%2A> method.  
+ The <xref:System.Web.HttpUtility.HtmlAttributeEncode%2A> method converts only double quotation marks ("), single quotation marks ('), ampersands (&), and left angle brackets (<) to equivalent character entities. It is considerably faster than the <xref:System.Web.HttpUtility.HtmlEncode%2A> method.  
   
  The string result from the <xref:System.Web.HttpUtility.HtmlAttributeEncode%2A> method should be used only for double-quoted attributes. Security issues might arise when using the <xref:System.Web.HttpUtility.HtmlAttributeEncode%2A> method with single-quoted attributes.  
   
@@ -1867,4 +1867,4 @@
       </Docs>
     </Member>
   </Members>
-</Type>
+</Type>