elided_named_lifetimes: add docs

Author: GrigorenkoPV

compiler/rustc_lint_defs/src/builtin.rs

@@ -1864,10 +1864,35 @@ declare_lint! {
 }
 
 declare_lint! {
-    /// TODO
+    /// The `elided_named_lifetimes` lint detects when an elided
+    /// lifetime ends up being a named lifetime, such as `'static`
+    /// or some lifetime parameter `'a`.
+    ///
+    /// ### Example
+    ///
+    /// ```rust,compile_fail
+    /// #![deny(elided_named_lifetimes)]
+    /// struct Foo;
+    /// impl Foo {
+    ///     pub fn get_mut(&'static self, x: &mut u8) -> &mut u8 {
+    ///         unsafe { &mut *(x as *mut _) }
+    ///     }
+    /// }
+    /// ```
+    ///
+    /// {{produces}}
+    ///
+    /// ### Explanation
+    ///
+    /// Lifetime elision is quite useful, because it frees you from having
+    /// to give each lifetime its own name, but sometimes it can produce
+    /// somewhat surprising resolutions. In safe code, it is mostly okay,
+    /// because the borrow checker prevents any unsoundness, so the worst
+    /// case scenario is you get a confusing error message in some other place.
+    /// But with `unsafe` code, such unexpected resolutions may lead to unsound code.
     pub ELIDED_NAMED_LIFETIMES,
     Warn,
-    "TODO"
+    "detects when an elided lifetime gets resolved to be `'static` or some named parameter"
 }
 
 declare_lint! {