Bug #117961 --flush only affects MyISAM storage engine, but RefMan doesn't describe it

Description:
Document describes --flush as

```
If ON, the server flushes (synchronizes) all changes to disk after each SQL statement. Normally, MySQL does a write of all changes to disk only after each SQL statement and lets the operating system handle the synchronizing to disk. See Section B.3.3.3, “What to Do If MySQL Keeps Crashing”. This variable is set to ON if you start mysqld with the --flush option.
```

It seems to affect whole MySQL server processing, but actually --flush only affects MyISAM storage engine, 

- --flush is handled as `myisam_flush` variable
  - https://github.com/mysql/mysql-server/blob/mysql-8.4.4/sql/sys_vars.cc#L2152-L2154 

- And `myisam_flush` is only used at storage/myisam
  - https://github.com/search?q=repo%3Amysql%2Fmysql-server%20myisam_flush&type=code

How to repeat:
See document described for --flush

https://dev.mysql.com/doc/refman/9.2/en/server-system-variables.html#sysvar_flush

Suggested fix:
Add "only for MyISAM" into the reference.

Hello tanaka-San,

Thank you for the report and feedback.

regards,
Umesh

Posted by developer:
 
Updated documentation as requested.
Thank you for the bug report.