Update Documentation Guide (#28)

# Update Documentation Guide

## ⚙️ Release Notes 
- Updates the documentation guide.


## 📝 Code of Conduct & Contributing Guidelines 

By submitting creating this pull request, you agree to follow our [Code
of
Conduct](https://github.com/StanfordSpezi/.github/blob/main/CODE_OF_CONDUCT.md)
and [Contributing
Guidelines](https://github.com/StanfordSpezi/.github/blob/main/CONTRIBUTING.md):
- [x] I agree to follow the [Code of
Conduct](https://github.com/StanfordSpezi/.github/blob/main/CODE_OF_CONDUCT.md)
and [Contributing
Guidelines](https://github.com/StanfordSpezi/.github/blob/main/CONTRIBUTING.md).

DOCUMENTATIONGUIDE.md

@@ -32,7 +32,12 @@ The documentation MUST use appropriate [GitHub markdown syntax](https://docs.git
 Related types within the module MUST be linked using [markdown links](https://docs.github.com/en/get-started/writing-on-github/getting-started-with-writing-and-formatting-on-github/basic-writing-and-formatting-syntax#links) or [DocC links](https://www.swift.org/documentation/docc/formatting-your-documentation-content#Link-to-Symbols-and-Other-Content).
 Essential external types SHOULD be linked to, e.g., the Apple Documentation like the [`@State`](https://developer.apple.com/documentation/swiftui/state) property wrapper.
 
-Code examples MUST use the proper syntax highlighting.
+Code examples MUST use proper syntax highlighting, e.g.,:
+````md
+```swift
+let name = "Paul"
+```
+````
 
 Elements like [notes and other asides](https://www.swift.org/documentation/docc/documenting-a-swift-framework-or-package) SHOULD be used.
 GitHub offers equal elements for README documents, e.g.:
@@ -41,6 +46,9 @@ GitHub offers equal elements for README documents, e.g.:
 > An important element ...
 ```
 
+You SHOULD use [`@_documentation(visibility: ...)`](https://github.com/apple/swift/blob/main/docs/ReferenceGuides/UnderscoredAttributes.md#_documentationvisibility-) to hide elements from the documentation that should not appear in the public documentation.
+It is RECOMMENDED to use `@_documentation(visibility: internal)` to an `@_exported import` statement to ensure that all public symbols of the imported module are not exposed in the documentation of the Spezi module.
+
 You can learn more about DocC by reading the documentation or watching the DocC Apple World Wide Developer Conference (WWDC) videos:
 - [swift.org - DocC Documentation](https://www.swift.org/documentation/docc/#)
 - [WWDC 2023 - Create rich documentation with Swift-DocC](https://developer.apple.com/wwdc23/10244)