Oh yes - also a note to the wiki page would be useful ;-)
https://mantisbt.org/wiki/doku.php/mantisbt:setting_up_mail_queuing

it would help noobs a lot to say very explicitly that the system admin has to set this up, and that it's not done mantis itself

Isn't "must be sent manually" very explicitly?
What should be added to these snippets from Admin Guide?

emails are queued and must be sent manually by executing scripts/send_emails.
...
The script's execution is typically handled by a cron job or a scheduler task, set to run every few minutes. 

Sorry if I’m sounding picky – these are tiny issues - but removing pebbles from the installation path should help to improve uptake :-)

Isn't "must be sent manually" very explicitly?

Apparently not ! I read it, but I didn’t understand… (if you understand already, it’s quite clear :-D ). Honestly, I was assuming that mantis code would be setting up the crontab for me.

What should be added to these snippets from Admin Guide?

It needs to be very explicit :-

emails are queued by mantis. An external utility must be set up by the system admin to execute the mantis-provided script send_mail.php at regular intervals.
...
Typically this utility is provided by a cron job or a scheduler task, which calls "php <install path>/mantis/scripts/send_mail.php.

@Timo thanks for suggesting an improvement to the documentation. I propose to revise the Admin Guide as follows:

$g_email_send_using_cronjob

Use a cron job or task scheduler to send emails.

When OFF (default), emails are sent as soon as an action is performed. The user will have to wait for MantisBT to process the email queue after each page load.

It is recommended to set this to ON. In that case, an external utility, typically a cron job or a scheduler task, must be set up by the system admin to execute the provided script scripts/send_emails.php at regular intervals (e.g. every few minutes). This script can only be run from the CLI, not from the web interface, for security reasons.

Note
Processing the email queue can take a significant amount of time on systems generating large numbers of notifications, when the queue contains many undeliverable emails or when mail delivery is slow. Enabling this option can significantly improve MantisBT's performance.

Good for you ?

Yes :-D