Fix config.sample and preview provider

[mmattel]

Description

When writing docs for owncloud/docs#2735 (Document the support for SGI Images) I ran into the issue that:

• Update in config.sample.php to reflect the necessary changes
  (which will then be transported via a config-to-docs run
• The PreviewManager did not had all necessary comments respectively had outdated information (there is no more an install possible on windows)

No code changes, text only!

Related Issue

• Fixes Document the support for SGI Images docs#2735
• Based on: Add sgi Previews #37758

Motivation and Context

Fix missing and wrong information

How Has This Been Tested?

With my own productive instance

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Database schema changes (next release will require increase of minor version instead of patch)
• Breaking change (fix or feature that would cause existing functionality to change)
• Technical debt
• Tests only (no source changes)

Checklist:

• Code changes
• Unit tests added
• Acceptance tests added
• Documentation ticket raised:
• Changelog item, see TEMPLATE

[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
No Duplication information