Improved helptext of how to send mail via the systems mail subsystem

[apio-sys]

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Description

Adds a help field on the notification setup screen for SMTP to make it clearer for those using local MTA.

Fixes #2793

Type of change

Please delete any options that are not relevant.

• User interface (UI)
• Other

Checklist

• My code follows the style guidelines of this project
• I ran ESLint and other linters for modified files
• I have performed a self-review of my own code and tested it
• I have commented my code, particularly in hard-to-understand areas (including JSDoc for methods)
• My changes generates no new warnings
• My code needed automated testing. I have added them (this is optional task)

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[CommanderStorm]

What do you think about this text instead?

This changes

• localhost is styled like code
• adds a link to wikipedia, for those don't know what an MTA is and just want to get mails set up
• changes the translation key to be basically the same to help our translators

[CommanderStorm]

Thanks for the bugfix!

All changes in this PR are small and uncontroversial ⇒ merging with junior maintainer approval