Add long error explanation for E0724

Minor refactoring

Minor refactoring

Update src/librustc_error_codes/error_codes/E0724.md
Co-authored-by: NDavid Wood <Q0KPU0H1YOEPHRY1R2SN5B5RL@david.davidtw.co>

Update src/librustc_error_codes/error_codes/E0724.md
Co-authored-by: NDavid Wood <Q0KPU0H1YOEPHRY1R2SN5B5RL@david.davidtw.co>

Update src/librustc_error_codes/error_codes/E0724.md
Co-authored-by: NDavid Wood <Q0KPU0H1YOEPHRY1R2SN5B5RL@david.davidtw.co>

Minor refactoring

src/librustc_error_codes/error_codes.rs

@@ -409,6 +409,7 @@
 E0719: include_str!("./error_codes/E0719.md"),
 E0720: include_str!("./error_codes/E0720.md"),
 E0723: include_str!("./error_codes/E0723.md"),
+E0724: include_str!("./error_codes/E0724.md"),
 E0725: include_str!("./error_codes/E0725.md"),
 E0727: include_str!("./error_codes/E0727.md"),
 E0728: include_str!("./error_codes/E0728.md"),
@@ -615,7 +616,6 @@
     E0717, // rustc_promotable without stability attribute
 //  E0721, // `await` keyword
     E0722, // Malformed `#[optimize]` attribute
-    E0724, // `#[ffi_returns_twice]` is only allowed in foreign functions
     E0726, // non-explicit (not `'_`) elided lifetime in unsupported position
 //  E0738, // Removed; errored on `#[track_caller] fn`s in `extern "Rust" { ... }`.
     E0755, // `#[ffi_pure]` is only allowed on foreign functions

src/librustc_error_codes/error_codes/E0724.md

+`#[ffi_returns_twice]` was used on non-foreign function.
+
+Erroneous code example:
+
+```compile_fail,E0724
+#![feature(ffi_returns_twice)]
+#![crate_type = "lib"]
+
+#[ffi_returns_twice] // error!
+pub fn foo() {}
+```
+
+`#[ffi_returns_twice]` can only be used on foreign function declarations.
+For example, we might correct the previous example by declaring
+the function inside of an `extern` block.
+
+```
+#![feature(ffi_returns_twice)]
+
+extern {
+   #[ffi_returns_twice] // ok!
+   pub fn foo();
+}
+```

src/test/ui/ffi_returns_twice.stderr

@@ -6,3 +6,4 @@ LL | #[ffi_returns_twice]
 
 error: aborting due to previous error
 
+For more information about this error, try `rustc --explain E0724`.