[class mods] Fix grammar to prohibit "mixin" on mixin applications. (#2847)

* [class mods] Fix grammar to prohibit "mixin" on mixin applications.

It looks like the prose (which I think is correct) got out of sync with
the grammar. This fixes that.

I also tweaked the Markdown formatting a bit because I'm a little
obsessive. I'm sorry.

* Update accepted/future-releases/class-modifiers/feature-specification.md

Co-authored-by: Erik Ernst <[email protected]>

* Update accepted/future-releases/class-modifiers/feature-specification.md

Co-authored-by: Erik Ernst <[email protected]>

* Wrap logically.

---------

Co-authored-by: Erik Ernst <[email protected]>

Author: munificent

accepted/future-releases/class-modifiers/feature-specification.md

@@ -4,7 +4,7 @@ Author: Bob Nystrom, Lasse Nielsen
 
 Status: Accepted
 
-Version 1.4
+Version 1.5
 
 Experiment flag: class-modifiers
 
@@ -523,10 +523,10 @@ Many combinations don't make sense:
     A `sealed mixin class`  does not provide any significant extra
     functionality over a `sealed mixin`, you can replace `extends MixinClass`
     with `with Mixin`, so a `sealed mixin class` is not allowed.*
-*   `interface` and `final` classes would prevent a mixin class from being used as a
-    superclass or mixin outside of its library. _Like for `sealed`, an `interface` or `final`
-    `mixin class` is not allowed, and an `interface` or `final` `mixin` declaration is
-    recommended instead._
+*   `interface` and `final` classes would prevent a mixin class from being used
+    as a superclass or mixin outside of its library. *Like for `sealed`, an
+    `interface mixin class` and `final mixin class` are not allowed, and
+    `interface mixin` and `final mixin` declaration are recommended instead.*
 *   `mixin` as a modifier can obviously only be applied to a `class`
     declaration, which makes it also a `mixin` declaration.
 *   `mixin` as a modifier cannot be applied to a mixin-application `class`
@@ -560,16 +560,17 @@ The remaining valid combinations and their capabilities are:
 The grammar is:
 
 ```
-classDeclaration  ::= classModifiers 'class' identifier typeParameters?
-                      superclass? interfaces?
+classDeclaration  ::= (classModifiers | mixinClassModifiers) 'class' typeIdentifier
+                      typeParameters? superclass? interfaces?
                       '{' (metadata classMemberDeclaration)* '}'
                       | classModifiers 'class' mixinApplicationClass
 
 classModifiers    ::= 'sealed'
                     | 'abstract'? ('base' | 'interface' | 'final')?
-                    | 'abstract'? 'base'? 'mixin'
 
-mixinDeclaration  ::= mixinModifier? 'mixin' identifier typeParameters?
+mixinClassModifiers ::= 'abstract'? 'base'? 'mixin'
+
+mixinDeclaration  ::= mixinModifier? 'mixin' typeIdentifier typeParameters?
                       ('on' typeNotVoidList)? interfaces?
                       '{' (metadata classMemberDeclaration)* '}'
 
@@ -770,7 +771,8 @@ Define a *trivial generative constructor* to be a generative constructor that:
 
 *   declares no parameters,
 
-*   has no initializer list (no `: ...` part, so no asserts or initializers, and no super constructor invocation),
+*   has no initializer list (no `: ...` part, so no asserts or initializers, and
+    no super constructor invocation),
 
 *   has no body (only `;`), and
 
@@ -933,6 +935,11 @@ needed.
 
 ## Changelog
 
+1.5
+
+- Fix mixin application grammar to match prose where `mixin` can't be applied
+  to a mixin application class.
+
 1.4
 
 - Update rules to close loopholes on classes that don't want to expose