Bug #121642 innodb_stats_auto_recalc threshold is based on estimated row count, not actual row count

Description:
The documentation states that when both innodb_stats_persistent and innodb_stats_auto_recalc are ON, statistics are recalculated automatically once more than 10% of the rows in a table have been changed.

8.4 documentation (the same text appears in 9.7):
https://dev.mysql.com/doc/refman/8.4/en/innodb-persistent-stats.html
> The innodb_stats_auto_recalc variable, which is enabled by default, controls whether statistics are calculated automatically when a table undergoes changes to more than 10% of its rows. 

Strictly speaking, however, the recalculation is triggered when the number of changed rows exceeds 10% of the *estimated* number of rows in the table, not the actual row count.

How to repeat:
# Check the server version and system variables
mysql> select version();
+--------------+
| version()    |
+--------------+
| 8.4.11-debug |
+--------------+
1 row in set (0.00 sec)

mysql> show variables like 'innodb_stats_%';
+--------------------------------------+-------------+
| Variable_name                        | Value       |
+--------------------------------------+-------------+
| innodb_stats_auto_recalc             | ON          |
| innodb_stats_include_delete_marked   | OFF         |
| innodb_stats_method                  | nulls_equal |
| innodb_stats_on_metadata             | OFF         |
| innodb_stats_persistent              | ON          |
| innodb_stats_persistent_sample_pages | 20          |
| innodb_stats_transient_sample_pages  | 8           |
+--------------------------------------+-------------+
7 rows in set (0.01 sec)

# Create an InnoDB table

mysql> use test;
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A
Database changed

mysql> CREATE TABLE stat_test (i int primary key) ENGINE=InnoDB, STATS_PERSISTENT=1,  STATS_AUTO_RECALC=1;
Query OK, 0 rows affected (0.04 sec)

mysql> insert into stat_test values (1), (10), (100);
Query OK, 3 rows affected (0.01 sec)
Records: 3  Duplicates: 0  Warnings: 0

# Update the rows in the table (to trigger automatic statistics recalculation)
mysql> update stat_test set i = i + 1;

At this point, I obtained the following stack trace with gdb:

row_update_statistics_if_needed(dict_table_t * table) (/home/ec2-user/mysql8411/mysql-server/storage/innobase/row/row0mysql.cc:1117)
row_update_for_mysql_using_upd_graph(const byte * mysql_rec, row_prebuilt_t * prebuilt) (/home/ec2-user/mysql8411/mysql-server/storage/innobase/row/row0mysql.cc:2427)
row_update_for_mysql(const byte * mysql_rec, row_prebuilt_t * prebuilt) (/home/ec2-user/mysql8411/mysql-server/storage/innobase/row/row0mysql.cc:2450)
ha_innobase::update_row(ha_innobase * const this, const uchar * old_row, uchar * new_row) (/home/ec2-user/mysql8411/mysql-server/storage/innobase/handler/ha_innodb.cc:10080)
handler::ha_update_row(handler * const this, const uchar * old_data, uchar * new_data) (/home/ec2-user/mysql8411/mysql-server/sql/handler.cc:8128)
Sql_cmd_update::update_single_table(Sql_cmd_update * const this, THD * thd) (/home/ec2-user/mysql8411/mysql-server/sql/sql_update.cc:1000)
Sql_cmd_update::execute_inner(Sql_cmd_update * const this, THD * thd) (/home/ec2-user/mysql8411/mysql-server/sql/sql_update.cc:1827)
Sql_cmd_dml::execute(Sql_cmd_dml * const this, THD * thd) (/home/ec2-user/mysql8411/mysql-server/sql/sql_select.cc:782)
mysql_execute_command(THD * thd, bool first_level) (/home/ec2-user/mysql8411/mysql-server/sql/sql_parse.cc:3672)
dispatch_sql_command(THD * thd, Parser_state * parser_state) (/home/ec2-user/mysql8411/mysql-server/sql/sql_parse.cc:5406)
dispatch_command(THD * thd, const COM_DATA * com_data, enum_server_command command) (/home/ec2-user/mysql8411/mysql-server/sql/sql_parse.cc:2136)
do_command(THD * thd) (/home/ec2-user/mysql8411/mysql-server/sql/sql_parse.cc:1465)
handle_connection(void * arg) (/home/ec2-user/mysql8411/mysql-server/sql/conn_handler/connection_handler_per_thread.cc:304)
pfs_spawn_thread(void * arg) (/home/ec2-user/mysql8411/mysql-server/storage/perfschema/pfs.cc:3067)
libc.so.6!start_thread
libc.so.6!clone3

row_update_statistics_if_needed() obtains the number of rows in the table by calling dict_table_get_n_rows():

https://github.com/mysql/mysql-server/blob/mysql-8.4.11/storage/innobase/row/row0mysql.cc#...
  n_rows = dict_table_get_n_rows(table);

  if (dict_stats_is_persistent_enabled(table)) {
    if (counter > n_rows / 10 /* 10% */
        && dict_stats_auto_recalc_is_enabled(table)) {
      dict_stats_recalc_pool_add(table);
      table->stat_modified_counter = 0;
    }
    return;
  }

dict_table_get_n_rows() is defined as follows, and it returns an estimate of the number of rows in the table:

https://github.com/mysql/mysql-server/blob/mysql-8.4.11/storage/innobase/include/dict0dict...
/** Gets the approximately estimated number of rows in the table.
 @return estimated number of rows */
static inline uint64_t dict_table_get_n_rows(
    const dict_table_t *table) /*!< in: table */
{
  ut_ad(table->stat_initialized);

  return (table->stat_n_rows);
}

Therefore, the threshold that triggers automatic statistics recalculation is based on the estimated number of rows in the table, not on the exact row count.

Suggested fix:
Please update the documentation to make it clear that automatic recalculation is triggered when changes exceed 10% of the **estimated** number of rows in the table.

For example:
The innodb_stats_auto_recalc variable, which is enabled by default, controls whether statistics are calculated automatically when a table undergoes changes to more than 10% of its rows, based on the estimated number of rows in the table.