docs(derive): Ensure magic attributes are searchable

clap_derive/CONTRIBUTING.md

@@ -9,3 +9,5 @@ See the [clap-wide CONTRIBUTING.md](../CONTRIBUTING.md).  This will contain `cla
 - Prefer substituting variable names to avoid problems with `macro_rules`, see [#2823](https://github.com/clap-rs/clap/pull/2823).
 - Prefer `::std::result::Result` and `::std::option::Option`, see [#3092](https://github.com/clap-rs/clap/pull/3092).
 - Put whitespace between `#quoted #variables`.
+- New "magic" attributes must be documented in the [derive reference](../src/_derive.rs)
+  - If there is no related builder method, a `#![doc(alias = "")]` should also be added

src/_derive/mod.rs

@@ -485,6 +485,20 @@
 //!   ([example][_tutorial#testing])
 //! - Always remember to [document](#doc-comments) args and commands with `#![deny(missing_docs)]`
 
+// Point people here that search for attributes that don't exist in the derive (a subset of magic
+// attributes)
+#![doc(alias = "skip")]
+#![doc(alias = "verbatim_doc_comment")]
+#![doc(alias = "flatten")]
+#![doc(alias = "external_subcommand")]
+#![doc(alias = "subcommand")]
+#![doc(alias = "rename_all")]
+#![doc(alias = "rename_all_env")]
+#![doc(alias = "default_value_t")]
+#![doc(alias = "default_values_t")]
+#![doc(alias = "default_value_os_t")]
+#![doc(alias = "default_values_os_t")]
+
 pub mod _tutorial;
 #[doc(inline)]
 pub use crate::_cookbook;