Clarify note about supported target types in "Defining test functions" article (#1291)

Author: stmontgomery

Sources/Testing/Testing.docc/DefiningTests.md

@@ -25,8 +25,9 @@ contains the test:
 import Testing
 ```
 
-- Note: Only import the testing library into a test target. Importing the
-  testing library into an application, library, or binary target isn't
+- Note: Only import the testing library into a test target or library meant for
+  test targets. Importing the testing library into a target intended for
+  distribution such as an application, app library, or executable target isn't
   supported or recommended. Test functions aren't stripped from binaries when
   building for release, so logic and fixtures of a test may be visible to anyone
   who inspects a build product that contains a test function.