Fix 4227: Improve documentation for --quiet --check (#4236)

Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>

CHANGES.md

@@ -50,6 +50,7 @@
 
 ### Documentation
 
+- Note what happens when `--check` is used with `--quiet` (#4236)
 <!-- Major changes to documentation and policies. Small docs changes
      don't need a changelog entry. -->
 

docs/usage_and_configuration/the_basics.md

@@ -183,6 +183,9 @@ Don't write the files back, just return the status. _Black_ will exit with:
 - code 1 if some files would be reformatted; or
 - code 123 if there was an internal error
 
+If used in combination with `--quiet` then only the exit code will be returned, unless
+there was an internal error.
+
 ```console
 $ black test.py --check
 All done! ✨ 🍰 ✨