update docs

Author: Jesse Haigh

Sources/docc/DocCDocumentation.docc/formatting-your-documentation-content.md

@@ -143,6 +143,24 @@ add a new line and terminate the code listing by adding another three backticks:
 instead of tabs so that DocC preserves the indentation when compiling your 
 documentation.
 
+#### Formatting Code Listings
+
+You can add a copy-to-clipboard button to a code listing by including the copy
+option after the name of the programming language for the code listing:
+
+```swift, copy
+struct Sightseeing: Activity {
+    func perform(with sloth: inout Sloth) -> Speed {
+        sloth.energyLevel -= 10
+        return .slow
+    }
+}
+```
+
+This renders a copy button in the top-right cotner of the code listing in
+generated documentation. When clicked, it copies the contents of the code
+block to the clipboard.
+
 DocC uses the programming language you specify to apply the correct syntax 
 color formatting. For the example above, DocC generates the following: