Adjust documentation for compatibility with 2021

This also adjusts the lint docs generation to accept (and ignore) an allow
attribute, rather than expecting the documentation to be immediately followed by
the lint name.

compiler/rustc_lint/src/array_into_iter.rs

@@ -13,7 +13,7 @@ declare_lint! {
     ///
     /// ### Example
     ///
-    /// ```rust
+    /// ```rust,edition2018
     /// # #![allow(unused)]
     /// [1, 2, 3].into_iter().for_each(|n| { *n; });
     /// ```

compiler/rustc_lint/src/builtin.rs

@@ -1686,7 +1686,7 @@ declare_lint! {
     ///
     /// ### Example
     ///
-    /// ```rust
+    /// ```rust,edition2018
     /// let x = 123;
     /// match x {
     ///     0...100 => {}

compiler/rustc_lint/src/non_fmt_panic.rs

@@ -18,7 +18,7 @@ declare_lint! {
     ///
     /// ### Example
     ///
-    /// ```rust,no_run
+    /// ```rust,no_run,edition2018
     /// panic!("{}");
     /// panic!(123);
     /// ```