Even if upstream is verified as a documentation bug, this does not seem to be the case for me. 

Description:
alter table... auto_increment=1 does not work as expected in case the auto_increment column is modified after table creation.

When the column is modified to auto_increment, the alter table... auto_increment=1 command does not reset the auto_increment value to the current maximum AUTO_INCREMENT column value plus one.
How to repeat:
Create a table without auto_increment

mysql> CREATE TABLE `test` (
    -> `id` int NOT NULL,
    -> PRIMARY KEY (`id`)
    -> ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci ROW_FORMAT=DYNAMIC;
Query OK, 0 rows affected (0.01 sec)

Insert a record to this table

mysql> insert into test set id=3;
Query OK, 1 row affected (0.00 sec)

Modified the column to make it auto_incrment with ALTER

mysql> alter table test modify id int not null auto_increment;
Query OK, 1 row affected (0.02 sec)
Records: 1  Duplicates: 0  Warnings: 0

Then reset the auto_incrment with ALTER TABLE..AUTO_INCREMENT=1

mysql> alter table test modify id int not null auto_increment;
Query OK, 1 row affected (0.02 sec)
Records: 1  Duplicates: 0  Warnings: 0

Then insert data into this table and duplicate entry error occur

mysql>  insert into test set id=null;
Query OK, 1 row affected (0.01 sec)

mysql>  insert into test set id=null;
Query OK, 1 row affected (0.01 sec)

mysql> select * from test;

3 rows in set (0.00 sec)

mysql>  insert into test set id=null;
ERROR 1062 (23000): Duplicate entry '3' for key 'test.PRIMARY'