Docs: Add note on --exclude about possibly verbose regex (#4145)

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

docs/usage_and_configuration/the_basics.md

@@ -268,6 +268,11 @@ recursive searches. An empty value means no paths are excluded. Use forward slas
 directories on all platforms (Windows, too). By default, Black also ignores all paths
 listed in `.gitignore`. Changing this value will override all default exclusions.
 
+If the regular expression contains newlines, it is treated as a
+[verbose regular expression](https://docs.python.org/3/library/re.html#re.VERBOSE). This
+is typically useful when setting these options in a `pyproject.toml` configuration file;
+see [Configuration format](#configuration-format) for more information.
+
 #### `--extend-exclude`
 
 Like `--exclude`, but adds additional files and directories on top of the default values