MultiScoping: search/delete can be scoped to a list of tenants

New method:
    Mongoid::Multitenancy.set_tenants current_tenant,
                                      secondary_tenant1,
                                      secondary_tenant2,
                                      ...

Sets the first argument as the `current_tenant` of the Thread,
which will be used for creation and/or validation purposes

Sets the `scoping_tenants` of the Thread as the entire list of tenants
(including the current_tenant), which will be used as the default_scope

Signed-off-by: Pranav Chaturvedi <pranav.g10@gmail.com>
added specs for MultiScoped Search/Delete

Signed-off-by: Pranav Chaturvedi <pranav.g10@gmail.com>
Update the documentation: Multi-Scoping

Signed-off-by: Pranav Chaturvedi <pranav.g10@gmail.com>