Update settings.rst: Admin privileges

[Rello]

Add info on Admin privileges

[joshtrichards]

Looks like we have some significant overlap between the Basic concepts->Settings and Digging deeper->Settings sections. And IDelegatedSettings is covered a bit in Basics already.

• https://docs.nextcloud.com/server/latest/developer_manual/basics/setting.html
• https://docs.nextcloud.com/server/latest/developer_manual/digging_deeper/settings.html

I wonder if we should take this opportunity to see if there's really any need for the Digging deeper version for Settings? And if there is still, maybe reduce some of the overlap and/or have them reference each other to minimize redundancy?

[kesselb]

I second Josh's suggestion.

[blizzz]

Please don't forget to set milestone and labels when opening a PR 😃