Docs for Allow public link sharing only for certain groups

[phil-davis]

WHAT Needs to be Documented?

PR owncloud/core#38980 adds admin sharing settings that allow public link sharing to be restricted to only users that are members of one of a list of groups.

WHERE Does This Need To Be Documented (Link)?

https://github.com/owncloud/docs/blob/master/modules/admin_manual/pages/configuration/files/file_sharing_configuration.adoc

WHY Should This Change Be Made?

New feature - needs to be documented.

(Optional) What Type Of Content Change Is This?

• New Content Addition
• Old Content Deprecation
• Existing Content Simplification
• Bug Fix to Existing Content

(Optional) Which Manual Does This Relate To?

• Admin Manual
• Developer Manual
• User Manual
• Android
• iOS
• Branded Clients
• Desktop Client
• Other

This feature will be in a release after 10.8 - whenever the next release happens after the PR is merged.

[AlexAndBear]

Edge-Case (currently, could change):

• Enable the setting Only certain groups are allowed to create public links -> don't set any groups.
• Create public share will still be possible for everyone (if no other rule interferes)