Clean up E0724 explanation

GuillaumeGomez · GuillaumeGomez · commit d6b50d823062 · 2020-07-23T21:16:16.000+02:00
diff --git a/src/librustc_error_codes/error_codes/E0724.md b/src/librustc_error_codes/error_codes/E0724.md
@@ -1,4 +1,5 @@
-`#[ffi_returns_twice]` was used on non-foreign function.
+`#[ffi_returns_twice]` was used on something other than a foreign function
+declaration.
 
 Erroneous code example:
 
