Add docstrings to IHtmlSanitizer

src/Umbraco.Core/Security/IHtmlSanitizer.cs

@@ -2,6 +2,11 @@ namespace Umbraco.Core.Security
 {
     public interface IHtmlSanitizer
     {
+        /// <summary>
+        /// Sanitizes HTML
+        /// </summary>
+        /// <param name="html">HTML to be sanitized</param>
+        /// <returns>Sanitized HTML</returns>
         string Sanitize(string html);
     }
 }