add custom strategies to property_test macro docs

Author: cameron1024

proptest-macro/src/lib.rs

@@ -35,7 +35,7 @@ mod property_test;
 /// of setting up the test harness and generating input values, allowing the user to focus
 /// on writing the test logic.
 ///
-/// # Attributes
+/// ## Attributes
 ///
 /// The `property_test` macro can take an optional `config` attribute, which allows you to
 /// customize the configuration of the `proptest` runner.
@@ -48,6 +48,22 @@ mod property_test;
 ///     assert_eq!(x, x);
 /// }
 /// ```
+///
+/// ## Custom strategies
+///
+/// By default, [`property_test`] will use the `Arbitrary` impl for parameters. However, you can
+/// provide a custom `Strategy` with `#[strategy = <expr>]` on an argument:
+///
+/// ```
+/// #[property_test]
+/// fn foo(#[strategy = "[0-9]*"] s: String) {
+///     for c in s.chars() {
+///         assert!(c.is_numeric());
+///     }
+/// }
+/// ```
+/// Multiple `#[strategy = <expr>]` attributes on an argument are not allowed.
+///
 #[proc_macro_attribute]
 pub fn property_test(attr: TokenStream, item: TokenStream) -> TokenStream {
     property_test::property_test(attr.into(), item.into()).into()