The question of whether or not to include the '&' came up in this comment 📌 Correct $suggestions documentation in hook_theme_suggestions_alter() Fixed in a core issue. The current Drupal standard for @param → does not mention parameters passed by reference.
My searching didn't uncover an answer although there is a similar question on stackoverflow.
If we adopted this change, the Drupal Project would benefit by ...
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Closed: duplicate
Coding Standards
I’d vote against including it, and it also doesn’t seem to be used in Core (or, a grand total of four times). But being explicit about it and adding it to the standards cannot hurt either way, I’d say.
This is a duplicate of an older issue #1368794: Using '&' in doxygen @param documentation → .
Closing this a duplicate.