Actually the documentation is correct in current state.
When you use the contain matcher, the order of rows is ignored

Then I'd say the sentence is not clear enough; I'm not able to parse it, and even after your comment I'm not sure what is the meaning of this note. Perhaps it can be reworded?

I guess it could be reworded to say that when using a contain matcher, the order of data is irrelevant and they are compared similar way when unordered matcher is used.
Certainly the word if is not a right one to use it in this scenario in my opinion.

I think I got it. The two lines:

join_by matcher is much faster on performing data comparison, consider using join_by over unordered

and

contain matcher is not considering order of compared data-sets. Using unordered makes no difference (it's default)

are two separate sentences. The first line is missing a period at the end. Am I right?

If so, please check the updated commit.