[ci skip] Add a comment to clarify the value of zend_call_info->num_args (#10812)

This surprised me a couple of times already, and it probably surprised
others too. Add a comment to clarify.

Author: nielsdos

Zend/Optimizer/zend_call_graph.h

@@ -38,7 +38,7 @@ struct _zend_call_info {
 	bool               send_unpack;  /* Parameters passed by SEND_UNPACK or SEND_ARRAY */
 	bool               named_args;   /* Function has named arguments */
 	bool               is_prototype; /* An overridden child method may be called */
-	int                     num_args;
+	int                     num_args;	/* Number of arguments, excluding named and variadic arguments */
 	zend_send_arg_info      arg_info[1];
 };