Grammar rule *parameter_modifier* disallows valid V7 constructs (#1209)

* fix grammar and add constraint

* allow both modifier orders

Co-authored-by: Nigel-Ecma <[email protected]>

* add punctuation

---------

Co-authored-by: Nigel-Ecma <[email protected]>

Author: RexJaeschke

standard/classes.md

@@ -2213,7 +2213,8 @@ default_argument
 
 parameter_modifier
     : parameter_mode_modifier
-    | 'this'
+    | 'this' parameter_mode_modifier?
+    | parameter_mode_modifier? 'this'
     ;
 
 parameter_mode_modifier
@@ -3162,8 +3163,9 @@ class Customer
 
 When the first parameter of a method includes the `this` modifier, that method is said to be an ***extension method***. Extension methods shall only be declared in non-generic, non-nested static classes. The first parameter of an extension method is restricted, as follows:
 
-- It may only be an input parameter if it has a value type
-- It may only be a reference parameter if it has a value type or has a generic type constrained to struct
+- It may only be an input parameter if it has a value type.
+- It may only be a reference parameter if it has a value type or has a generic type constrained to struct.
+- It shall not be an output parameter.
 - It shall not be a pointer type.
 
 > *Example*: The following is an example of a static class that declares two extension methods: