Add documentation about the edition flag

Author: Munksgaard

src/doc/rustdoc/src/documentation-tests.md

@@ -323,6 +323,22 @@ compiles, then the test will fail. However please note that code failing
 with the current Rust release may work in a future release, as new features
 are added.
 
+```text
+/// Only runs on the 2018 edition.
+///
+/// ```edition2018
+/// let result: Result<i32, ParseIntError> = try {
+///     "1".parse::<i32>()?
+///         + "2".parse::<i32>()?
+///         + "3".parse::<i32>()?
+/// };
+/// ```
+```
+
+`edition2018` tells `rustdoc` that the code sample should be compiled the 2018
+edition of Rust. Similarly, you can specify `edition2015` to compile the code
+with the 2015 edition.
+
 ## Syntax reference
 
 The *exact* syntax for code blocks, including the edge cases, can be found