Document use cases and locking behavior for OPTIMIZE TABLE with innodb_defragment=1

Using MariaDB 10.3, I've just tested the Defragmentation for the first time.

I added to my.cnf:
innodb_defragment = 1
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And then, after restart, I tried "OPTIMIZE TABLE xxxx" on a frequently used table (lots of INSERTs per second), to see how it goes.

I see that pretty much every INSERT query was getting stuck waiting for locks to be released.
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This doesn't seem to be an improvement over the original OPTIMIZE which creates a new temp table and then swaps, and consequently the table file will be much shorter.

While the new OPTIMIZE using defragmentation locks the table and the file won't get shorter.
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Is it intentional that OPTIMIZE using defragmentation locks the table?

MDEV-9155 is marked as Fixed/Closed since April 2017, so I assumed this would not lock anymore.
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Thanks!