Improve Documentation to describe how to work with indexes

[yashka713]

For example, I have a User model with Devise inside. Devise generates a batch of columns and indexes for them:

Columns:

• less_password_token
• reset_password_token
• invitation_token

And indexes:

• index_users_on_less_password_token
• index_users_on_reset_password_token
• index_users_on_invitation_token

Currently, documentation does not clearly describe how to work with indexes. So could you copy this to the database configuration example?

User:
  index_users_on_less_password_token:
    UniqueIndexChecker:
      enabled: false
  index_users_on_reset_password_token:
    UniqueIndexChecker:
      enabled: false
  index_users_on_invitation_token:
    UniqueIndexChecker:
      enabled: false