Clarify use of ActiveRecord where in search_import

[JasonBarnabe]

The readme explains search_import as such:

To eager load associations, use the search_import scope.
(...)
By default, all records are indexed. To control which records are indexed, use the should_index? method together with the search_import scope.

Now an obvious extrapolation here would be to add where to search_import to eliminate records that should not be indexed. This is a suggestion also present in some issues like #1503.

I've tested this out and it does work to eliminate records, but there is one major caveat - if a record is already indexed, then updated such that the where in search_import no longer matches it, the record remains in the index.

For example:

class User
  searchkick
  scope :search_import, -> { where.not(name: 'Jason') }

  def should_index?
    name != 'Jason'
  end
end

user = User.create(name: 'Fred')
# (User gets indexed)

user.update(name: 'Jason')
# (User remains in index!)

Removing search_import in the above example fixes the issue.

I'm not sure if this is intended functionality, but if it is, this should be documented - for example by saying that where should only added to search_import if the record will never be updated so that it shouldn't be indexed.