Add the new flag for the encryption command to the docs

What Needs to be Documented?

The changes from this PR owncloud/core#35606 need to be documented.

The Release Notes for 10.2.1 cover it pretty well.
https://doc.owncloud.com/server/admin_manual/release_notes.html#changes-in-10-2-1

Where Does This Need To Be Documented?

Admin Manual

Why Should This Change Be Made? (Optional)

Important to know for automated deployments

What Type Of Content Change Is This? (Optional)

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

Which Manual Does This Relate To? (Optional)

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