Document `default_scope` changes

These changes are made at https://github.com/rails/rails/commit/94924dc32baf78f13e289172534c2e71c9c8cade and discussed in https://github.com/rails/rails/issues/13875.

Closes https://github.com/rails/rails/issues/13875
Closes https://github.com/rails/rails/pull/13870

cc @fxn