Add example to lint docs

compiler/rustc_lint_defs/src/builtin.rs

@@ -1228,12 +1228,40 @@ declare_lint! {
 }
 
 declare_lint! {
-    /// The missing_fragment_specifier warning is issued when an unused pattern in a
+    /// The `missing_fragment_specifier` lint is issued when an unused pattern in a
     /// `macro_rules!` macro definition has a meta-variable (e.g. `$e`) that is not
     /// followed by a fragment specifier (e.g. `:expr`).
     ///
     /// This warning can always be fixed by removing the unused pattern in the
     /// `macro_rules!` macro definition.
+    ///
+    /// ### Example
+    ///
+    /// ```rust,compile_fail
+    /// macro_rules! foo {
+    ///    () => {};
+    ///    ($name) => { };
+    /// }
+    ///
+    /// fn main() {
+    ///    foo!();
+    /// }
+    /// ```
+    ///
+    /// {{produces}}
+    ///
+    /// ### Explanation
+    ///
+    /// To fix this, remove the unused pattern from the `macro_rules!` macro definition:
+    ///
+    /// ```rust
+    /// macro_rules! foo {
+    ///     () => {};
+    /// }
+    /// fn main() {
+    ///     foo!();
+    /// }
+    /// ```
     pub MISSING_FRAGMENT_SPECIFIER,
     Deny,
     "detects missing fragment specifiers in unused `macro_rules!` patterns",