Chromium exercise solutions (#1564)

The main thing in this CL is a link to exercise solutions but there are
a couple of other very minor tweaks.

src/SUMMARY.md

@@ -278,6 +278,7 @@
   - [Keeping crates up to date](chromium/adding-third-party-crates/keeping-up-to-date.md)
   - [Exercise](exercises/chromium/third-party.md)
 - [Bringing it together - Exercise](exercises/chromium/bringing-it-together.md)
+- [Exercise Solutions](exercises/chromium/solutions.md)
 
 
 # Bare Metal: Morning

src/chromium/interoperability-with-cpp/example-bindings.md

@@ -1,7 +1,7 @@
 # Example bindings
 
 CXX requires that the whole C++/Rust boundary is declared in `cxx::bridge`
-"modules" inside `.rs` source code.
+modules inside `.rs` source code.
 
 ```rust,ignore
 {{#include ../../../third_party/cxx/book/snippets.rs:cxx_overview}}
@@ -11,6 +11,10 @@ CXX requires that the whole C++/Rust boundary is declared in `cxx::bridge`
 
 Point out:
 
+* Although this looks like a regular Rust `mod`, the `#[cxx::bridge]`
+  procedural macro does complex things to it. The generated code is quite
+  a bit more sophisticated - though this does still result in a `mod` called
+  `ffi` in your code.
 * Native support for C++'s `std::unique_ptr` in Rust
 * Native support for Rust slices in C++
 * Calls from C++ to Rust, and Rust types (in the top part)

src/chromium/interoperability-with-cpp/using-cxx-in-chromium.md

@@ -33,7 +33,7 @@ compromise the safety of Rust's own data layouts. Presence of _too many_
 such a keyword, and is [controversial][1], but strictly, bringing any foreign
 code into a Rust binary can cause unexpected behavior from Rust's perspective.
 
-The narrow answer lies in the diagram at the top of this page --- behind the
+The narrow answer lies in the diagram at the top of [this page][2] --- behind the
 scenes, CXX generates Rust `unsafe` and `extern "C"` functions just like
 we did manually in the previous section.
 
@@ -41,3 +41,4 @@ we did manually in the previous section.
 
 [0]: https://source.chromium.org/chromium/chromium/src/+/main:base/containers/span_rust.h;l=21
 [1]: https://steveklabnik.com/writing/the-cxx-debate
+[2]: ../interoperability-with-cpp.md

src/exercises/chromium/solutions.md

@@ -0,0 +1,6 @@
+# Exercise Solutions
+
+Solutions to the Chromium exercises can be found in
+[this series of CLs][0].
+
+[0]: https://chromium-review.googlesource.com/c/chromium/src/+/5096560