Add suggested good cases in docs for lifetimes lint

Author: dario23

clippy_lints/src/lifetimes.rs

@@ -24,9 +24,15 @@ declare_clippy_lint! {
     ///
     /// **Example:**
     /// ```rust
+    /// // Bad: unnecessary lifetime annotations
     /// fn in_and_out<'a>(x: &'a u8, y: u8) -> &'a u8 {
     ///     x
     /// }
+    ///
+    /// // Good
+    /// fn elided(x: &u8, y: u8) -> &u8 {
+    ///     x
+    /// }
     /// ```
     pub NEEDLESS_LIFETIMES,
     complexity,
@@ -46,9 +52,15 @@ declare_clippy_lint! {
     ///
     /// **Example:**
     /// ```rust
+    /// // Bad: unnecessary lifetimes
     /// fn unused_lifetime<'a>(x: u8) {
     ///     // ..
     /// }
+    ///
+    /// // Good
+    /// fn no_lifetime(x: u8) {
+    ///     // ...
+    /// }
     /// ```
     pub EXTRA_UNUSED_LIFETIMES,
     complexity,