Update docs for `ANN401` (#6009)

Part of #5803

crates/ruff/src/rules/flake8_annotations/helpers.rs

@@ -30,7 +30,7 @@ pub(super) fn match_function_def(
             body,
             decorator_list,
         ),
-        _ => panic!("Found non-FunctionDef in match_name"),
+        _ => panic!("Found non-FunctionDef in match_function_def"),
     }
 }
 

crates/ruff/src/rules/flake8_annotations/rules/definition.rs

@@ -376,7 +376,7 @@ impl Violation for MissingReturnTypeClassMethod {
 }
 
 /// ## What it does
-/// Checks that an expression is annotated with a more specific type than
+/// Checks that function arguments are annotated with a more specific type than
 /// `Any`.
 ///
 /// ## Why is this bad?
@@ -399,9 +399,23 @@ impl Violation for MissingReturnTypeClassMethod {
 ///     ...
 /// ```
 ///
+/// ## Known problems
+///
+/// Type aliases are unsupported and can lead to false positives.
+/// For example, the following will trigger this rule inadvertently:
+/// ```python
+/// from typing import Any
+///
+/// MyAny = Any
+///
+///
+/// def foo(x: MyAny):
+///     ...
+/// ```
+///
 /// ## References
 /// - [PEP 484](https://www.python.org/dev/peps/pep-0484/#the-any-type)
-/// - [`typing.Any`](https://docs.python.org/3/library/typing.html#typing.Any)
+/// - [Python documentation: `typing.Any`](https://docs.python.org/3/library/typing.html#typing.Any)
 /// - [Mypy: The Any type](https://mypy.readthedocs.io/en/stable/kinds_of_types.html#the-any-type)
 #[violation]
 pub struct AnyType {