Merge pull request #9630 from robertjwhitney/update_collection_docs

Update documentation for collection<< to reflect behavior for new parent objects.

activerecord/lib/active_record/associations.rb

@@ -1024,7 +1024,7 @@ module ActiveRecord
       # [collection<<(object, ...)]
       #   Adds one or more objects to the collection by setting their foreign keys to the collection's primary key.
       #   Note that this operation instantly fires update sql without waiting for the save or update call on the
-      #   parent object.
+      #   parent object, unless the parent object is a new record.
       # [collection.delete(object, ...)]
       #   Removes one or more objects from the collection by setting their foreign keys to +NULL+.
       #   Objects will be in addition destroyed if they're associated with <tt>dependent: :destroy</tt>,
@@ -1433,7 +1433,7 @@ module ActiveRecord
       #   Adds one or more objects to the collection by creating associations in the join table
       #   (<tt>collection.push</tt> and <tt>collection.concat</tt> are aliases to this method).
       #   Note that this operation instantly fires update sql without waiting for the save or update call on the
-      #   parent object.
+      #   parent object, unless the parent object is a new record.
       # [collection.delete(object, ...)]
       #   Removes one or more objects from the collection by removing their associations from the join table.
       #   This does not destroy the objects.