Describe unsafe usage of invert_where method [ci skip]

[pocke]

invert_where method has been added by #40249.
This pull request adds documentation of unsafe usage of invert_where.

As #40249 (comment), this method is dangerous.
Because it inverts all conditions before invert_where call.

For example

require "bundler/inline"

gemfile(true) do
  source "https://rubygems.org"

  git_source(:github) { |repo| "https://github.com/#{repo}.git" }

  gem "activerecord", github: 'rails/rails', ref: 'be630f7d1d948c274bbb67ab6ed2148c539c9b8c'
  gem "activemodel", github: 'rails/rails', ref: 'be630f7d1d948c274bbb67ab6ed2148c539c9b8c'
  gem "activesupport", github: 'rails/rails', ref: 'be630f7d1d948c274bbb67ab6ed2148c539c9b8c'
  gem "sqlite3"
end

require "active_record"

ActiveRecord::Base.establish_connection(adapter: "sqlite3", database: ":memory:")

ActiveRecord::Schema.define do
  create_table :users, force: true do |t|
    t.string :role
    t.boolean :accepted
    t.boolean :locked
  end
end

class User < ActiveRecord::Base
  scope :active, -> { where(accepted: true, locked: false) }
  scope :inactive, -> { active.invert_where } # Do not attempt it
end
                                                                  
puts User.where(role: 'admin').inactive.to_sql
# SELECT "users".* FROM "users" WHERE NOT ("users"."role" = 'admin' AND "users"."accepted" = 1 AND "users"."locked" = 0)

On the last line, User.where(role: 'admin').inactive inverts where(role: 'admin') unexpectedly.

But the current documentation only describes a single where method call case. So we can understand how where(...).invert_where works, but cannot understand how where(...).where(...).invert_where works from the document.

This behavior can become a bug, so the documentation should describe the behavior.

---

By the way, I'm wondering if adding it should be reverted. Because it is too dangerous IMO. I will not use this method for my project to avoid problems.
But I think it needs the document to describe the risk at least.

I also opened an issue to rubocop-rails for this method. rubocop/rubocop-rails#470