PHPDoc: Replace non-existing return type exit with never-returns
Summary:
`exit` is not a valid @return type in PHPDoc per https://docs.phpdoc.org/guide/references/phpdoc/types.html.
Go for `never-returns` which seems the best compromise according to https://phpstan.org/writing-php-code/phpdoc-types#bottom-type and https://stackoverflow.com/questions/31672862/phpdoc-function-exits
Test Plan: Read the code.
Reviewers: O1 Blessed Committers, valerio.bozzolan
Reviewed By: O1 Blessed Committers, valerio.bozzolan
Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Differential Revision: https://we.phorge.it/D26096