tweak docs for rustdoc's --show-coverage

src/doc/rustdoc/src/unstable-features.md

@@ -445,9 +445,13 @@ Some methodology notes about what rustdoc counts in this metric:
 
 * Rustdoc will only count items from your crate (i.e. items re-exported from other crates don't
   count).
-* Since trait implementations can inherit documentation from their trait, separate totals are given
-  both with and without trait implementations.
-* Inherent impl blocks are not counted, even though their doc comments are displayed, because the
-  common pattern in Rust code is to write all inherent methods into the same impl block.
+* Docs written directly onto inherent impl blocks are not counted, even though their doc comments
+  are displayed, because the common pattern in Rust code is to write all inherent methods into the
+  same impl block.
+* Items in a trait implementation are not counted, as those impls will inherit any docs from the
+  trait itself.
 * By default, only public items are counted. To count private items as well, pass
   `--document-private-items` at the same time.
+
+Public items that are not documented can be seen with the built-in `missing_docs` lint. Private
+items that are not documented can be seen with Clippy's `missing_docs_in_private_items` lint.