Document removed E0773.

Author: m-ou-se

compiler/rustc_error_codes/src/error_codes/E0773.md

@@ -0,0 +1,4 @@
+#### this error code is no longer emitted by the compiler.
+
+This was triggered when multiple macro definitions used the same
+`#[rustc_builtin_macro(..)]`. This is no longer an error.

compiler/rustc_error_codes/src/lib.rs

@@ -515,6 +515,7 @@ E0769: 0769,
 E0770: 0770,
 E0771: 0771,
 E0772: 0772,
+E0773: 0773,
 E0774: 0774,
 E0775: 0775,
 E0776: 0776,
@@ -687,6 +688,5 @@ E0804: 0804,
 //  E0723, // unstable feature in `const` context
 //  E0738, // Removed; errored on `#[track_caller] fn`s in `extern "Rust" { ... }`.
 //  E0744, // merged into E0728
-//  E0773, // Builtin macro defined more than once. This is no longer an error.
 //  E0776, // Removed; cmse_nonsecure_entry is now `C-cmse-nonsecure-entry`
 //  E0796, // unused error code. We use `static_mut_refs` lint instead.