Add warning about version matching to book

Author: dtolnay

book/src/build/bazel.md

@@ -15,6 +15,15 @@ $  cxxbridge src/bridge.rs --header > path/to/bridge.rs.h
 $  cxxbridge src/bridge.rs > path/to/bridge.rs.cc
 ```
 
+<div class="warning">
+
+**Important:** The version number of `cxxbridge-cmd` used for the C++ side of
+the binding must always be identical to the version number of `cxx` used for the
+Rust side. You must use some form of lockfile or version pinning to ensure that
+this is the case.
+
+</div>
+
 The CXX repo maintains working [Bazel] `BUILD.bazel` and [Buck2] `BUCK` targets
 for the complete blobstore tutorial (chapter 3) for your reference, tested in
 CI. These aren't meant to be directly what you use in your codebase, but serve

book/src/build/other.md

@@ -36,6 +36,16 @@ But the C++ side of the bindings needs to be generated. Your options are:
 - Or, build your own code generator frontend on top of the [cxx-gen] crate. This
   is currently unofficial and unsupported.
 
+<div class="warning">
+
+**Important:** The Rust side and C++ side of a binding must always be created
+using the same release of CXX. If using `cxxbridge-cmd` for the C++ side, the
+version number of `cxxbridge-cmd` must be identical to the version number of
+`cxx` used for the Rust side. If using `cxx-gen` for the C++ side, its patch
+number must be identical to the patch number of `cxx`.
+
+</div>
+
 [cxx-gen]: https://docs.rs/cxx-gen
 
 ### Compiling C++