Remove "mixin" from some positions in the @reopen text. (#2965)

* Remove "mixin" from some positions in the @reopen text.

Regarding reopened mixins and reopening mixins, I think @eernstg [worded it well](dart-archive/linter#4235 (comment))

> It is not possible to reopen a mixin or a mixin class: It has the modifier `base` or no modifier, and no subtype of a `base` declaration can exist without a modifier which is at least as strong as `base`.

Based on this, I made these changes:

* removed "or mixin" from the things which can be annotated with `@reopen`
* removed "or mixin" from the things which can be extended or mixed in and be modified with `interface` or `final`
* spread a sentence which has 7 "or"s in it 🤣 across two bullet points.

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

Co-authored-by: Kallen Tu <[email protected]>

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

Co-authored-by: Kallen Tu <[email protected]>

* wrap

---------

Co-authored-by: Kallen Tu <[email protected]>

Author: srawlins

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

@@ -901,12 +901,12 @@ Instead, it's a suggested part of the overall user experience of the feature.
 
 A metadata annotation `@reopen` is added to package [meta][] and a lint
 "implicit_reopen" is added to the [linter][]. When the lint is enabled, a lint
-warning is reported if a class or mixin is not annotated `@reopen` and it:
+warning is reported if a class is not annotated `@reopen` and it:
 
-*   Extends or mixes in a class, mixin, or mixin class marked `interface` or
-    `final` and is not itself marked `interface` or `final`,
-    or extends or mixes in a `sealed` declaration which itself
-    transitively extends or mixes in an `interface` or `final` declaration.
+*   extends a class marked `interface` or `final`
+    and is not itself marked `interface` or `final`, or
+*   extends a `sealed` class which itself transitively extends a class marked
+    `interface` or `final`.
 
 [meta]: https://pub.dev/packages/meta
 [linter]: https://dart.dev/guides/language/analysis-options#enabling-linter-rules