default sort on `many_many` relation table isn't clearly documented

Affected Version

5.0.0

Description

I have a BlogPost DataObject that has many_many relation with TaxonomyTerm DataObject through BlogPostTaxonomyTerm as below:

private static array $many_many = [
    'Terms' => [
        'through' => BlogPostTaxonomyTerm::class,
        'from' => 'Parent',
        'to' => 'TaxonomyTerm',
    ],
];

There is a Sort field on the TaxonomyTerm DataObject and a separate Sort field on the many_many through relation DataObject BlogPostTaxonomyTerm.

I want to sort the linked TaxonomyTerms on the BlogPost based on the Sort value on the BlogPostTaxonomyTerm table. However, I am not able to do that. The Sorting result is always determined by the Sort value on the TaxonomyTerm DataObject instead.

Here is an example, I have the following fixture file for unit test:

SilverStripe\Taxonomy\TaxonomyTerm:
  term1:
    Name: term1
    Sort: 1
  term2:
    Name: term2
    Sort: 2
  term3:
    Name: term3
    Sort: 3

App\Pages\BlogPost:
  page1:
    Title: 'Blog Post 1'
    PublishDate: '2023-06-20 15:55:50'

App\Taxonomy\ThroughRelations\BlogPostTaxonomyTerm:
  blogPostTerm1:
    Parent: =>App\Pages\BlogPost.page1
    TaxonomyTerm: =>SilverStripe\Taxonomy\TaxonomyTerm.term3
    Sort: 1
  blogPostTerm2:
    Parent: =>App\Pages\BlogPost.page1
    TaxonomyTerm: =>SilverStripe\Taxonomy\TaxonomyTerm.term2
    Sort: 2
  blogPostTerm3:
    Parent: =>App\Pages\BlogPost.page1
    TaxonomyTerm: =>SilverStripe\Taxonomy\TaxonomyTerm.term1
    Sort: 3

And I want to test the first term of the App\Pages\BlogPost.page1.
The expected result is the first term should be "term3", however the actual first term is "term1".

After I add the following config to the BlogPostTaxonomyTerm many_many relation, the Sorting works as expected.

 private static string $default_sort = '"BlogPostTaxonomyTerm"."Sort"';

Search the developer documentation for the many_many relation and default_sort configuration described in the issue. Document that sorting uses the related TaxonomyTerm by default and explain how to configure sorting on the through relation, using the provided BlogPostTaxonomyTerm example. Done means a newcomer can find and understand the required configuration.