Document xjs_load() useage.

docs/websrvr.txt

@@ -451,4 +451,12 @@ for(i=1; i<=10; i++) {
 </html>
 
 
+4.2 XJS-specific commands
+-------------------------
+The following JS commands are available to XJS files only.
+xjs_load(filename)
+	Includes the specified file as though its contents were part of the
+	current file at the current position.  Acts as though the current xjs
+	tag is closed before the file and re-opened after it.
+
 /* End of file */