n.a.

mysql_upgrade issues a lots of commands, but they don't change anything. For example, there's a bunch of ALTER TABLE statements that add columns to system tables. They fail if these columns already exist.

May be the manual could better say "changes nothing" instead of "does nothing".

I do not agree with your comment "but they don't change anything" except the general log is lying and the queries were not really executed. But then this would be another bug report.

Logically you might be correct. But physically this might have an impact. For example in some stupid edge cases where I have zillions of users in my mysql.user table or similar.

I counted 259 DDL and 69 DML statements. And this can take quite a while where my database is or should not be responsive. further those statements are counted in the com_ counters and could confuse my alerting/monitoring.
Further this could lead to heavy unexpected load on my server.

So what mysql_upgrade does: it basically recreates the whole MySQL DD! IMHO documentation should reflect what technically is happening...

Changed text to "changes nothing" as per Sergei's suggestion. Most of the statements are of the "CREATE TABLE IF NOT EXISTS" nature or as Sergei pointed out will fail if the column already exists so it's hardly recreating the entire DD.