Fill out documentation for new lint

Author: djkoloski

compiler/rustc_lint_defs/src/builtin.rs

@@ -3774,8 +3774,21 @@ declare_lint! {
 }
 
 declare_lint! {
- #[doc(hidden)]
- /// Added for testing unsable lints; perma-unstable.
+ /// The `test_unstable_lint` lint tests unstable lints and is perma-unstable.
+ ///
+ /// ### Example
+ ///
+ /// ```
+ /// #![allow(test_unstable_lint)]
+ /// ```
+ ///
+ /// {{produces}}
+ ///
+ /// ### Explanation
+ ///
+ /// In order to test the behavior of unstable traits, a permanently-unstable
+ /// lint is required. This lint can be used to trigger warnings and errors
+ /// from the compiler related to unstable lints.
  pub TEST_UNSTABLE_LINT,
  Deny,
  "this unstable lint is only for testing",