update AR::ImmutableRelation documentation [ci skip]

activerecord/lib/active_record/errors.rb

@@ -193,6 +193,20 @@ module ActiveRecord
 
   end
 
+  # Raised when a relation cannot be mutated because it's already loaded.
+  #
+  #   class Task < ActiveRecord::Base
+  #   end
+  #
+  #   relation = Task.all
+  #   relation.loaded? # => true
+  #
+  #   # where! will try to mutate the relation, but this will fails because it's loaded
+  #   relation.where!(title: 'TODO')
+  #   # => ActiveRecord::ImmutableRelation
+  #
+  #   relation.limit!(5)
+  #   # => ActiveRecord::ImmutableRelation
   class ImmutableRelation < ActiveRecordError
   end