Auto merge of #30074 - jonas-schievink:macro-doc, r=sanxiyn

Fixes #17616

New docs for `panic!`:
```rust
macro_rules! panic {
    () => { ... };
    ($msg:expr) => { ... };
    ($fmt:expr, $($arg:tt)+) => { ... };
}
```

New docs for `assert!`:
```rust
macro_rules! assert {
    ( $ cond : expr ) => { ... };
    (
$ cond : expr , $ ( $ arg : tt ) + ) => { ... };
}
```
<sup>not pretty, but at least it's not worse 😂

src/test/rustdoc/macros.rs

@@ -0,0 +1,20 @@
+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// @has macros/macro.my_macro!.html //pre 'macro_rules! my_macro {'
+// @has - //pre '() => { ... };'
+// @has - //pre '($a:tt) => { ... };'
+// @has - //pre '($e:expr) => { ... };'
+#[macro_export]
+macro_rules! my_macro {
+    () => [];
+    ($a:tt) => ();
+    ($e:expr) => {};
+}