Mention SE review "Trying it out" instructions as a reason for using package copy in docs (#1660)

This adds a sentence to
[`Distributions.md`](https://github.com/swiftlang/swift-testing/blob/main/Documentation/Distributions.md)
acknowledging that one acceptable use of the package copy of Swift
Testing is when trying out new features or APIs during a Swift evolution
proposal review.

### 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.

Author: stmontgomery

Documentation/Distributions.md

@@ -79,7 +79,9 @@ because it has several downsides:
 The primary reason Swift Testing is available to be used as a Swift package is
 to support its own development. The core contributors regularly develop Swift
 Testing by building it locally as a package, following workflows described in
-[Contributing][], and its CI builds that way as well.
+[Contributing][], and its CI builds that way as well. Additionally, when proposals
+for Swift Testing features or APIs undergo Swift evolution review, the "Trying it
+out" instructions typically recommend using the package for convenience.
 
 It's also sometimes helpful to use Swift Testing as a package in order to
 validate how changes made to the testing library will impact supporting tools,