add cargo doc --document-private-items

Author: steveklabnik

_posts/2018-09-13-Rust-1.29.md

@@ -169,6 +169,13 @@ will now try to fix up lockfiles that have been corrupted by a `git
 merge`](https://github.com/rust-lang/cargo/pull/5831/). You can pass
 `--locked` to disable this behavior.
 
+`cargo doc` has also grown a new flag:
+[`--document-private-items`](https://github.com/rust-lang/cargo/pull/5543).  By
+default, `cargo doc` only documents public things, as the docs it produces are
+intended for end-users. But if you're working on your own crate, and you have
+internal documentation for yourself to refer to, `--document-private-items`
+will generate docs for all items, not just public ones.
+
 See the [detailed release notes][notes] for more.
 
 ## Contributors to 1.29.0