Update unstable lint docs to include required feature attributes

Author: Aditya-PS-05

compiler/rustc_lint/src/unqualified_local_imports.rs

@@ -12,6 +12,7 @@ declare_lint! {
     /// ### Example
     ///
     /// ```rust,edition2018
+    /// #![feature(unqualified_local_imports)]
     /// #![warn(unqualified_local_imports)]
     ///
     /// mod localmod {

compiler/rustc_lint_defs/src/builtin.rs

@@ -2671,6 +2671,7 @@ declare_lint! {
     /// ### Example
     ///
     /// ```rust
+    /// #![feature(strict_provenance_lints)]
     /// #![warn(fuzzy_provenance_casts)]
     ///
     /// fn main() {
@@ -2714,6 +2715,7 @@ declare_lint! {
     /// ### Example
     ///
     /// ```rust
+    /// #![feature(strict_provenance_lints)]
     /// #![warn(lossy_provenance_casts)]
     ///
     /// fn main() {
@@ -4033,6 +4035,7 @@ declare_lint! {
     /// ### Example
     ///
     /// ```rust
+    /// #![feature(test_unstable_lint)]
     /// #![allow(test_unstable_lint)]
     /// ```
     ///