The file name and its location are incorrect. There is more information about it in #1722 and https://github.com/PHPCSStandards/PHP_CodeSniffer/blob/master/.github/CONTRIBUTING.md#writing-sniff-documentation.

The command below should display the documentation, but it doesn't because of that:

vendor/bin/phpcs --generator=Text --standard=WordPress --sniffs=WordPress.WP.AlternativeFunctions

The correct path should be WordPress/Docs/WP/AlternativeFunctionsStandard.xml

The indentation for code samples should start at the beginning of the line. Otherwise, code samples with multiple lines will have incorrect indentation.

Also, per the #1722 description, each line within the code sample should be a maximum of 48 characters.

Please use <em> tags to highlight the "good" and the "bad" bits in the code examples. More information in the description of #1722.

Correct me if I'm missing something, but as far as I can test, the use of header() does not trigger the WordPress.WP.AlternativeFunctions. I created a file with the contents of this invalid example and run PHPCS, but got no errors:

$ cat test.inc 
<?php

header( "Location: $url" );
$ vendor/bin/phpcs --standard=WordPress test.inc -s --sniffs=WordPress.WP.AlternativeFunctions
$

I haven't tested all the other <code_comparison> blocks, but on a quick look, it seems to me file_get_contents() does trigger this sniff, but the others don't. Now I'm starting to question myself if I'm checking the correct sniff. This is the one that I'm looking: