Add prose documentation for file/directory attachments. (#1440)

Document how to attach a file or directory to a test.

### Checklist:

- [x] Code and documentation should follow the style of the [Style
Guide](https://github.com/apple/swift-testing/blob/main/Documentation/StyleGuide.md).
- [x] If public symbols are renamed or modified, DocC references should
be updated.

---------

Co-authored-by: jgmcnutt <[email protected]>

Author: grynspan

Sources/Overlays/_Testing_Foundation/Attachments/Attachment+URL.swift

@@ -47,7 +47,7 @@ extension Attachment where AttachableValue == _AttachableURLWrapper {
   ///
   /// When you call this initializer and pass it the URL of a file, it reads or
   /// maps the contents of that file into memory. When you call this initializer
-  /// and pass it the URL of a directory, it creates a temporary zip file of the
+  /// and pass it the URL of a directory, it creates a temporary ZIP file of the
   /// directory before reading or mapping it into memory. These operations may
   /// take some time, so this initializer suspends the calling task until they
   /// are complete.

Sources/Testing/Testing.docc/Attachments.md

@@ -76,6 +76,29 @@ extension SalesReport: Encodable, Attachable {}
   your test target imports the [Foundation](https://developer.apple.com/documentation/foundation)
   module.
 
+### Attach files and directories
+
+If you have a file you want to save as an attachment, you can attach it using
+its file URL. The testing library needs to read or map the file before attaching it
+to your test, and those operations can fail, so you need to explicitly create an
+instance of ``Attachment`` before you record it.
+
+```swift
+import Foundation
+
+@Test func `sales report adds up`() async throws {
+  let salesReport = await generateSalesReport()
+  try salesReport.validate()
+  let salesReportURL = try salesReport.save()
+  let attachment = try await Attachment(contentsOf: salesReportURL)
+  Attachment.record(attachment)
+}
+```
+
+You can also attach a directory to a test using its file URL. When you attach a
+directory to a test, the testing library creates a ZIP file containing the
+directory's contents, then attaches that ZIP file in place of the directory.
+
 ### Attach images
 
 You can attach instances of the following system-provided image types to a test: