Description

I want to explicitly document how are dictionaries added.
Specifically, values in the second dictionary will override values in the first one, but the order of keys is kept.
For example, (a: 1, b: 2) + (a: 3) is (a: 3, b: 2), not (a: 1, b: 2) or (b: 2, a: 3).
See IndexMap::extend in indexmap::map - Rust.

Besides, maybe add a link from dictionary.insert.

Discord:

Dictionaries guarantee insertion order

At present, it looks like the following sentence is the only documentation. And there's no test for + and join.

A bit relates to #1258.