Revert "Added documentation to explain what happens if you don't set any values for the dependent option on has_many"

This reverts commit 820a677d.

Added documentation to explain what happens if you don't set any values for the dependent option on has_many

The main reason for this is that I want to separate the code that does
the mutating from the code that does the cloning.

* Remove unused association_class method.
* Remove a unnecessary assignment.
* Move @updated to BelongsToAssociation that only reference this instance variable.
* Reset @stale_state at the reset method. I think this place is right place.

So a NullRelation (Relation#none) is chainable with database methods.

Closes #5202 and #919

Revert "Documents that ActiveRecord instances also support []/[]= methods for reading/writing attributes"

This reverts commit a49fe6ec.

Reason: The existing doc about using self[:attribute] is clear enough.

Only write_attribute and read_attribute were documented and there was no mention to this alternative.

The #relation method gets called in four places and the return value was instantly cloned in three of them. The only place that did not clone was ActiveRecord::Scoping::Default::ClassMethods#unscoped. This introduced a bug described in #5667 and should really clone the relation, too. This means all four places would clone the relation, so it doesn't make a lot of sense caching it in the first place.

The four places with calls to relations are:

activerecord/lib/active_record/scoping/default.rb:110:in `block in build_default_scope'"
activerecord/lib/active_record/scoping/default.rb:42:in `unscoped'"
activerecord/lib/active_record/scoping/named.rb:38:in `scoped'"
activerecord/lib/active_record/scoping/named.rb:52:in `scope_attributes'"