`mail_domain` clarification re: SMTP EHLO

[monochromec]

☑️ Resolves

• Fixes misleading mail_domain config parameter description (cf. https://github.com/nextcloud/server/issues/43955 ).

[monochromec]

Fixed malformed link.

[joshtrichards]

This is already noted in the actual E-mail configuration chapter:

https://docs.nextcloud.com/server/latest/admin_manual/configuration_server/email_configuration.html#configuring-an-smtp-server

[!NOTE] The overwrite.cli.url parameter from config.php will be used for the SMTP EHLO.

I don't think it needs to be included in the config.sample.php personally (as proposed in this PR) since it's a fairly obscure customization, but if one is going to do that... the config.sample.php in this repository is actually pulled from the server repository here. The PR would have to be against that.

[github-actions[bot]]

Hello there, Thank you so much for taking the time and effort to create a pull request to our Nextcloud project.

We hope that the review process is going smooth and is helpful for you. We want to ensure your pull request is reviewed to your satisfaction. If you have a moment, our community management team would very much appreciate your feedback on your experience with this PR review process.

Your feedback is valuable to us as we continuously strive to improve our community developer experience. Please take a moment to complete our short survey by clicking on the following link: https://cloud.nextcloud.com/apps/forms/s/i9Ago4EQRZ7TWxjfmeEpPkf6

Thank you for contributing to Nextcloud and we hope to hear from you soon!