Merge pull request #19120 from in0rdr/feat/document-stderr-redirect

Document stderr redirection for shell_output tests

Author: MikeMcQuaid

docs/Formula-Cookbook.md

@@ -334,6 +334,14 @@ test do
 end
 ```
 
+* If the binary only writes to `stderr`, you can redirect `stderr` to `stdout` for assertions with `shell_output`. For example:
+
+```ruby
+test do
+  assert_match "Output on stderr", shell_output("#{bin}/formula-program 2>&1", 2)
+end
+```
+
 ### Manuals
 
 Homebrew expects to find manual pages in `#{prefix}/share/man/...`, and not in `#{prefix}/man/...`.