docs, make association autosave: true examples runnable. Closes #14700

[ci skip]

The examples are written in a way you expect them to be executable.
However one snippet assumed there to be two comments when only one
was created above.

The defined models did not extend `ActiveRecord::Base`

The example used `comments.last.mark_for_destruction`. This does no
longer load the whole collection but just the last record. It is
then refetcht on subsequent calls to `last`. This breaks the example.

activerecord/lib/active_record/autosave_association.rb

@@ -35,7 +35,7 @@ module ActiveRecord
   #
   # === One-to-one Example
   #
-  #   class Post
+  #   class Post < ActiveRecord::Base
   #     has_one :author, autosave: true
   #   end
   #
@@ -76,7 +76,7 @@ module ActiveRecord
   #
   # When <tt>:autosave</tt> is not declared new children are saved when their parent is saved:
   #
-  #   class Post
+  #   class Post < ActiveRecord::Base
   #     has_many :comments # :autosave option is not declared
   #   end
   #
@@ -95,20 +95,23 @@ module ActiveRecord
   # When <tt>:autosave</tt> is true all children are saved, no matter whether they
   # are new records or not:
   #
-  #   class Post
+  #   class Post < ActiveRecord::Base
   #     has_many :comments, autosave: true
   #   end
   #
   #   post = Post.create(title: 'ruby rocks')
   #   post.comments.create(body: 'hello world')
   #   post.comments[0].body = 'hi everyone'
-  #   post.save # => saves both post and comment, with 'hi everyone' as body
+  #   post.comments.build(body: "good morning.")
+  #   post.title += "!"
+  #   post.save # => saves both post and comments.
   #
   # Destroying one of the associated models as part of the parent's save action
   # is as simple as marking it for destruction:
   #
-  #   post.comments.last.mark_for_destruction
-  #   post.comments.last.marked_for_destruction? # => true
+  #   post.comments # => [#<Comment id: 1, ...>, #<Comment id: 2, ...]>
+  #   post.comments[1].mark_for_destruction
+  #   post.comments[1].marked_for_destruction? # => true
   #   post.comments.length # => 2
   #
   # Note that the model is _not_ yet removed from the database: