Add unittest example to rustdoc (#1276)

Fixes #1275

---------

Co-authored-by: Martin Geisler <martin@geisler.net>

src/basic-syntax/rustdoc.md

@@ -6,6 +6,11 @@ All language items in Rust can be documented using special `///` syntax.
 /// Determine whether the first argument is divisible by the second argument.
 ///
 /// If the second argument is zero, the result is false.
+///
+/// # Example
+/// ```
+/// assert!(is_divisible_by(42, 2));
+/// ```
 fn is_divisible_by(lhs: u32, rhs: u32) -> bool {
     if rhs == 0 {
         return false;  // Corner case, early return
@@ -18,6 +23,7 @@ The contents are treated as Markdown. All published Rust library crates are
 automatically documented at [`docs.rs`](https://docs.rs) using the
 [rustdoc](https://doc.rust-lang.org/rustdoc/what-is-rustdoc.html) tool. It is
 idiomatic to document all public items in an API using this pattern.
+Code snippets can document usage and will be used as unit tests.
 
 <details>