Describe full behaviour of Active Record's attribute query methods

Update the docs to more accurately describe how the Active Record attribute query methods behave ([applicable code here](https://github.com/rails/rails/blob/master/activerecord/lib/active_record/attribute_methods/query.rb)).
Present and blank are Rails terminology and are used freely throughout the docs. In principle I am :-1: on this edit. Maybe we could have a "vocabulary" section somewhere or something, I don't know, but don't feel like adding that kind of remark to every instance of "present" and "blank".

@sgrif agreed, and there are more rules in these predicates IIRC.