[stubtest] Improve allowlist docs with better example

docs/source/stubtest.rst

@@ -99,8 +99,71 @@ to mypy build errors". In this case, you will need to mitigate those errors
 before stubtest will run. Despite potential overlap in errors here, stubtest is
 not intended as a substitute for running mypy directly.
 
+Allowlist
+*********
+
 If you wish to ignore some of stubtest's complaints, stubtest supports a
-pretty handy allowlist system.
+pretty handy :option:`--allowlist` system.
+
+Let's say that you have this python module called ``ex``:
+
+.. code-block:: python
+
+   try:
+       import optional_expensive_dep
+   except ImportError:
+       optional_expensive_dep = None
+
+   first = 1
+   if optional_expensive_dep:
+       second = 2
+
+Let's say that you can't install ``optional_expensive_dep`` in CI for some reason,
+but you still want to include ``second: int`` in the stub file:
+
+.. code-block:: python
+
+    first: int
+    second: int
+
+In this case stubtest will correctly complain:
+
+.. code-block:: shell
+
+   error: ex.second is not present at runtime
+   Stub: in file /.../ex.pyi:2
+   builtins.int
+   Runtime:
+   MISSING
+
+   Found 1 error (checked 1 module)
+
+To fix this, you can add an ``allowlist`` entry:
+
+.. code-block: ini
+
+   # Allowlist entries in `allowlist.txt` file:
+
+   # Does not exist if `optional_expensive_dep` is not installed:
+   ex.second
+
+And now when running stubtest with ``--allowlist=allowlist.txt``,
+no errors will be generated anymore.
+
+Allowlists also support regular expressions.
+They might be useful to ignore many similar errors at once.
+If some CI workers have ``optional_expensive_dep`` installed stubtest will complain:
+
+.. code-block: ini
+
+   note: unused allowlist entry ex.second
+   Found 1 error (checked 1 module)
+
+Changing ``ex.second`` to be ``(ex\.second)?`` will make this error optional.
+And stubtest will not fail with and without ``optional_expensive_dep``.
+
+CLI
+***
 
 The rest of this section documents the command line interface of stubtest.
 
@@ -119,15 +182,15 @@ The rest of this section documents the command line interface of stubtest.
 .. option:: --allowlist FILE
 
     Use file as an allowlist. Can be passed multiple times to combine multiple
-    allowlists. Allowlists can be created with --generate-allowlist. Allowlists
-    support regular expressions.
+    allowlists. Allowlists can be created with :option:`--generate-allowlist`.
+    Allowlists support regular expressions.
 
     The presence of an entry in the allowlist means stubtest will not generate
     any errors for the corresponding definition.
 
 .. option:: --generate-allowlist
 
-    Print an allowlist (to stdout) to be used with --allowlist
+    Print an allowlist (to stdout) to be used with :option:`--allowlist`.
 
     When introducing stubtest to an existing project, this is an easy way to
     silence all existing errors.
@@ -141,17 +204,17 @@ The rest of this section documents the command line interface of stubtest.
 
     Note if an allowlist entry is a regex that matches the empty string,
     stubtest will never consider it unused. For example, to get
-    `--ignore-unused-allowlist` behaviour for a single allowlist entry like
+    ``--ignore-unused-allowlist`` behaviour for a single allowlist entry like
     ``foo.bar`` you could add an allowlist entry ``(foo\.bar)?``.
     This can be useful when an error only occurs on a specific platform.
 
 .. option:: --mypy-config-file FILE
 
-    Use specified mypy config file to determine mypy plugins and mypy path
+    Use specified mypy config *file* to determine mypy plugins and mypy path
 
 .. option:: --custom-typeshed-dir DIR
 
-    Use the custom typeshed in DIR
+    Use the custom typeshed in *DIR*
 
 .. option:: --check-typeshed