Document ignore_missing_stub in CONTRIBUTING.md (#11426)

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

CONTRIBUTING.md

@@ -202,13 +202,16 @@ supported:
 * `partial_stub` (optional): This field marks the type stub package as
   [partial](https://peps.python.org/pep-0561/#partial-stub-packages). This is for
   3rd-party stubs that don't cover the entirety of the package's public API.
-  In most cases, this field is identical to `ignore_missing_stub`.
 
 In addition, we specify configuration for stubtest in the `tool.stubtest` table.
 This has the following keys:
 * `skip` (default: `false`): Whether stubtest should be run against this
   package. Please avoid setting this to `true`, and add a comment if you have
   to.
+* `ignore_missing_stub`: When set to `true`, this will add the
+  `--ignore_missing_stub` option to the stubtest call. See
+  [tests/README.md](./tests/README.md) for more information. In most cases,
+  this field should be identical to `partial_stub`.
 * `apt_dependencies` (default: `[]`): A list of Ubuntu APT packages
   that need to be installed for stubtest to run successfully.
 * `brew_dependencies` (default: `[]`): A list of MacOS Homebrew packages

tests/README.md

@@ -186,7 +186,7 @@ but missing from the stub. However, this behaviour can be disabled using the
 
 If a distribution has `ignore_missing_stub = true` in the `[tool.stubtest]` section of its
 `tests/METADATA.toml` file, `stubtest_third_party.py` will test that distribution with the
-`--ignore-missing-stub option`. This indicates that the stubs for this distribution are
+`--ignore-missing-stub` option. This indicates that the stubs for this distribution are
 considered "incomplete".
 
 You can help make typeshed's stubs more complete by removing