docs: Document should_panic macro deviation

Author: skogseth

crates/libtest2/src/lib.rs

@@ -38,6 +38,7 @@
 //! - `#[test]` does not support all `Termination` types as return values,
 //!   only what [`IntoRunResult`] supports.
 //! - `#[ignore]` must come after the `#[test]` macro
+//! - `#[should_ignore]` must come after the `#[test]` macro
 //! - Output capture and `--no-capture`: simply not supported. The official
 //!   `libtest` uses internal `std` functions to temporarily redirect output.
 //!   `libtest` cannot use those, see also [libtest2#12](https://github.com/assert-rs/libtest2/issues/12)