Touch up examples in C++ bindings docs.

docs/cpp/classes_and_structs.md

@@ -23,6 +23,14 @@ Crubit will generate a struct with the same layout:
 <!--  class:Position -->
 
 
+Which can be used like this:
+
+```
+{{ #include ../../examples/cpp/trivial_struct/main.rs }}
+```
+<!--  function:main -->
+
+
 For an example of a Rust-movable class with a destructor, see
 [examples/cpp/trivial_abi_struct/](https://github.com/google/crubit/tree/main/examples/cpp/trivial_abi_struct/).
 

docs/cpp/enums.md

@@ -29,7 +29,15 @@ Crubit will generate the following bindings:
 ```
 {{ #include ../../examples/cpp/enum/example_generated.rs }}
 ```
-<!--  class:Color -->
+<!--  content:^([^/#\n]|#[^!]|///)[^\n]* -->
+
+
+Which can be used like this:
+
+```
+{{ #include ../../examples/cpp/enum/main.rs }}
+```
+<!--  function:main -->
 
 
 ## Why isn't it an `enum`?

docs/cpp/functions.md

@@ -37,6 +37,14 @@ calls into the corresponding FFI glue:
 <!--  function:add_two_integers -->
 
 
+Which can be used like this:
+
+```
+{{ #include ../../examples/cpp/function/main.rs }}
+```
+<!--  function:main -->
+
+
 ### Methods
 
 Given the following C++ header:
@@ -61,6 +69,14 @@ Crubit will generate the following bindings:
 <!--  content:"(?m)impl Bar.*?(^$|^    \})" -->
 
 
+Which can be used like this:
+
+```
+{{ #include ../../examples/cpp/method/main.rs }}
+```
+<!--  function:main -->
+
+
 ### `unsafe` functions {#unsafe}
 
 #### Which C++ functions are marked `unsafe` in Rust? {#unsafe-inference}
@@ -89,6 +105,14 @@ Crubit will generate the following bindings:
 <!--  content:^([^/\n])([^!\n]|$)[^\n]* -->
 
 
+Which can be used like this:
+
+```
+{{ #include ../../examples/cpp/unsafe_attributes/main.rs }}
+```
+<!--  function:main -->
+
+
 #### Correct usage of `unsafe` {#using-unsafe}
 
 Functions marked **`unsafe`** cannot be called outside of an `unsafe` block. In