Replace :disabled with nil

If there's a column named `disabled`, we'll still raise
`ActiveRecord::SubclassNotFound`. Setting `inheritance_column` to `nil`
works though.

I had mistakenly thought `:disabled` was the correct way to disable STI,
since this is what is done in [some tests][].

```ruby
Schema: cogs[ id, type, disabled, created_at, updated_at ]

class Cog < ApplicationRecord
  self.inheritance_column = nil
end

Cog.create! type: "Sprocket", disabled: true
=> #<Cog type: "Sprocket", disabled: "t">
```

[some tests]: https://github.com/rails/rails/blob/a4751751bced209550f38f48bb58b64406d475a5/activerecord/test/models/post.rb#L231

Author: stevepolitodesign

activerecord/lib/active_record/inheritance.rb

@@ -204,7 +204,7 @@ def sti_class_for(type_name)
           "Please rename this column if you didn't intend it to be used for storing the inheritance class " \
           "or overwrite #{name}.inheritance_column to use another column for that information. " \
           "If you wish to disable single-table inheritance for #{name} set " \
-          "#{name}.inheritance_column to :disabled."
+          "#{name}.inheritance_column to nil"
       end
 
       # Returns the value to be stored in the polymorphic type column for Polymorphic Associations.

activerecord/lib/active_record/model_schema.rb

@@ -148,9 +148,9 @@ module ModelSchema
     #     self.inheritance_column = 'zoink'
     #
     # If you wish to disable single-table inheritance altogether you can set
-    # +inheritance_column+ to +:disabled+
+    # +inheritance_column+ to +nil+
     #
-    #     self.inheritance_column = :disabled
+    #     self.inheritance_column = nil
 
     ##
     # :singleton-method: inheritance_column=

guides/source/association_basics.md

@@ -2937,12 +2937,12 @@ There may be cases (like when working with a legacy database) where you need to
 disable Single Table Inheritance altogether. Otherwise, you'll raise
 [`ActiveRecord::SubclassNotFound`][].
 
-This can be achieved by setting the [inheritance_column][] to `:disabled`.
+This can be achieved by setting the [inheritance_column][] to `nil`.
 
 ```ruby
 # Schema: vehicles[ id, type, created_at, updated_at ]
 class Vehicle < ApplicationRecord
-  self.inheritance_column = :disabled
+  self.inheritance_column = nil
 end
 
 Vehicle.create!(type: "Car")