This PR updates the `[mailer]` configuration snippet for Gmail:
- The `HELO_HOSTNAME` isn't required.
- The `USER` must not include the @gmail domain.
- `HOST` needs to be supplied, and the SMTP port number needs to be
appended to the URL.
I also added a note about the requirement to use App passwords instead
of your Google account password directly.
Co-authored-by: delvh <dev.lh@web.de>
