Document the fact that json! can panic.
/// Variables or expressions can be interpolated into the JSON literal. Any type
/// interpolated into an array element or object value must implement Serde's
/// `Serialize` trait, while any type interpolated into a object key must
-/// implement `Into<String>`.
+/// implement `Into<String>`. If the `Serialize` implementation of the
+/// interpolated type decides to fail, or if the interpolated type contains a
+/// map with non-string keys, the `json!` macro will panic.
///
/// ```rust
/// # #![allow(unused_variables)]
