Mention unnameable_types in unreachable_pub documentation.

This link makes sense because someone who wishes to avoid unusable `pub`
is likely, but not guaranteed, to be interested in avoiding unnameable
types.

Also fixed some grammar problems I noticed in the area.

Fixes #116604.

compiler/rustc_lint_defs/src/builtin.rs

@@ -4362,7 +4362,7 @@ declare_lint! {
     /// ### Explanation
     ///
     /// It is often expected that if you can obtain an object of type `T`, then
-    /// you can name the type `T` as well, this lint attempts to enforce this rule.
+    /// you can name the type `T` as well; this lint attempts to enforce this rule.
     /// The recommended action is to either reexport the type properly to make it nameable,
     /// or document that users are not supposed to be able to name it for one reason or another.
     ///