Adds info about (lack of) object safety to `iter` module documentation.
(c.f.
//! to mirror the unstable `std::ops::Try` with implementations for `Option` and
//! `Result`, where `Some`/`Ok` values will let those iterators continue, but
//! `None`/`Err` values will exit early.
+//!
+//! A note about object safety: It is currently _not_ possible to wrap
+//! a ParallelIterator (or any trait that depends on it) using a
+//! `Box<dyn ParallelIterator>` or other kind of dynamic allocation,
+//! because `ParallelIterator` is **not object-safe**.
+//! (This keeps the implementation simpler and allows extra optimizations.)
use self::plumbing::*;
use self::private::Try;
