Document the special code block annotations.

Author: ehuss

crates/semver-check/src/main.rs

@@ -7,6 +7,11 @@
 //! An example with the word "MINOR" at the top is expected to successfully
 //! build against the before and after. Otherwise it should fail. A comment of
 //! "// Error:" will check that the given message appears in the error output.
+//!
+//! The code block can also include the annotations:
+//! - `run-fail`: The test should fail at runtime, not compiletime.
+//! - `dont-deny`: By default tests have a `#![deny(warnings)]`. This option
+//!   avoids this attribute. Note that `#![allow(unused)]` is always added.
 
 use std::error::Error;
 use std::fs;