Document language settings for sharing via email

[VicDeo]

what needs to be documented:

Language used for public mail notifications for shared files

1. appears when Allow users to send mail notification for shared files is checked
2. default value is owner language. it means that email notification will be in the language of the user that shared an item via email.
3. it is possible to change this setting via CLI with
   php occ config:app:set core shareapi_public_notification_lang --value 'languageCode'
   where language code is 'de', 'ru', etc

[PVince81]

Estimate: 0.25md.

Should be enough to explain that this option exists and clarify what exactly it does.

[settermjd]

Sounds fair to me @PVince81. I'll have that done today.