Add pragma once to the documentation

doc/cc65.sgml

@@ -1681,6 +1681,16 @@ void somefunc2(int, char *);
   </verb></tscreen>
 
 
+<sect1><tt>#pragma once</tt><label="pragma-once"><p>
+
+    This pragma is used to prevent multiple inclusion of a header file.
+    If this pragma is preprocessed in a file, any posterior
+    <tt>#include<tt> which targets the current file will be ignored.
+
+    Two files are considered to be the same by this directive if they
+    share the same absolute path. In the case of symbolic links,
+    the canonical, resolved path is considered instead.
+
 
 <sect>Register variables<label id="register-vars"><p>