Add more examples to exclude files in addition to the defaults (#3070)

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

docs/usage_and_configuration/the_basics.md

@@ -259,10 +259,14 @@ expressions by Black. Use `[ ]` to denote a significant space character.
 line-length = 88
 target-version = ['py37']
 include = '\.pyi?$'
+# 'extend-exclude' excludes files or directories in addition to the defaults
 extend-exclude = '''
 # A regex preceded with ^/ will apply only to files and directories
 # in the root of the project.
-^/foo.py  # exclude a file named foo.py in the root of the project (in addition to the defaults)
+(
+  ^/foo.py    # exclude a file named foo.py in the root of the project
+  | *_pb2.py  # exclude autogenerated Protocol Buffer files anywhere in the project
+)
 '''
 ```