Remove incorrect documentation about build_assoc on has_one. This is …

…proven, for example, by test_successful_build_association in has_one_associations_test.rb

activerecord/lib/active_record/associations.rb

@@ -1021,8 +1021,7 @@ def has_many(association_id, options = {}, &extension)
       # [build_association(attributes = {})]
       #   Returns a new object of the associated type that has been instantiated
       #   with +attributes+ and linked to this object through a foreign key, but has not
-      #   yet been saved. <b>Note:</b> This ONLY works if an association already exists.
-      #   It will NOT work if the association is +nil+.
+      #   yet been saved.
       # [create_association(attributes = {})]
       #   Returns a new object of the associated type that has been instantiated
       #   with +attributes+, linked to this object through a foreign key, and that