Update the description of lint type_alias_bounds

Author: fmease

Diff for: compiler/rustc_lint/src/builtin.rs

@@ -1459,20 +1459,32 @@ declare_lint! {
     ///
     /// ### Explanation
     ///
-    /// The trait bounds in a type alias are currently ignored, and should not
-    /// be included to avoid confusion. This was previously allowed
-    /// unintentionally; this may become a hard error in the future.
+    /// Trait and lifetime bounds on generic parameters and in where clauses of
+    /// type aliases are not checked at usage sites of the type alias. Moreover,
+    /// they are not thoroughly checked for correctness at their definition site
+    /// either similar to the aliased type.
+    ///
+    /// This is a known limitation of the type checker that may be lifted in a
+    /// future edition. Permitting such bounds in light of this was unintentional.
+    ///
+    /// While these bounds may have secondary effects such as enabling the use of
+    /// "shorthand" associated type paths[^1] and affecting the default trait
+    /// object lifetime[^2] of trait object types passed to the type alias, this
+    /// should not have been allowed until the aforementioned restrictions of the
+    /// type checker have been lifted.
+    ///
+    /// Using such bounds is highly discouraged as they are actively misleading.
+    ///
+    /// [^1]: I.e., paths of the form `T::Assoc` where `T` is a type parameter
+    /// bounded by trait `Trait` which defines an associated type called `Assoc`
+    /// as opposed to a fully qualified path of the form `<T as Trait>::Assoc`.
+    /// [^2]: <https://doc.rust-lang.org/reference/lifetime-elision.html#default-trait-object-lifetimes>
     TYPE_ALIAS_BOUNDS,
     Warn,
     "bounds in type aliases are not enforced"
 }
 
-declare_lint_pass!(
-    /// Lint for trait and lifetime bounds in type aliases being mostly ignored.
-    /// They are relevant when using associated types, but otherwise neither checked
-    /// at definition site nor enforced at use site.
-    TypeAliasBounds => [TYPE_ALIAS_BOUNDS]
-);
+declare_lint_pass!(TypeAliasBounds => [TYPE_ALIAS_BOUNDS]);
 
 impl TypeAliasBounds {
     pub(crate) fn affects_object_lifetime_defaults(pred: &hir::WherePredicate<'_>) -> bool {