Adjust AssertPduSize checks for Reading multiple DataItems.

The limit calculations did not match what the send and parsing code expected.

sending request header seems to be 19 byte in general.

Also adjust XML comments somewhat, since max PDU really differs a lot between PLC types, from 240 to 960 afaik.

S7.Net/PlcSynchronous.cs

@@ -500,13 +500,11 @@ namespace S7.Net
         /// You have to create and pass a list of DataItems and you obtain in response the same list with the values.
         /// Values are stored in the property "Value" of the dataItem and are already converted.
         /// If you don't want the conversion, just create a dataItem of bytes. 
-        /// DataItems must not be more than 20 (protocol restriction) and bytes must not be more than 200 + 22 of header (protocol restriction).
+        /// The number of DataItems as well as the total size of the requested data can not exceed a certain limit (protocol restriction).
         /// </summary>
-        /// <param name="dataItems">List of dataitems that contains the list of variables that must be read. Maximum 20 dataitems are accepted.</param>
+        /// <param name="dataItems">List of dataitems that contains the list of variables that must be read.</param>
         public void ReadMultipleVars(List<DataItem> dataItems)
         {
-            //Snap7 seems to choke on PDU sizes above 256 even if snap7 
-            //replies with bigger PDU size in connection setup.
             AssertPduSizeForRead(dataItems);
 
             var stream = GetStreamIfAvailable();