Add an example showing the attribute in use.

Author: ehuss

src/attributes/coverage-instrumentation.md

@@ -19,6 +19,21 @@ There are two ways to use the coverage attribute:
 * `#[coverage(off)]` indicates that all functions within an item, recursively, should not be instrumented, unless specified by another attribute.
 * `#[coverage(on)]` (the default) indicates that all functions within an item, recursively, *should* be instrumented, unless specified by another attribute.
 
+```rust
+#[coverage(off)]
+fn example() {}
+
+struct S;
+
+#[coverage(off)]
+impl S {
+    #[coverage(on)]
+    fn function_with_coverage() {}
+
+    fn function_without_coverage() {}
+}
+```
+
 r[attributes.coverage.allowed-positions]
 The `coverage` attribute can only be controlled at the function level, but it can be applied to modules, `impl` blocks, or anything that can contain functions.