ci: test docs with --document-private-items

This commit updates the `cargo doc` CI job to run the tool with
`--document-private-items`, testing that cargo doc comments on crate
internal items are free from error. These comments, while not part of
the public API docs, can be helpful for crate developers and should be
kept tidy where they exist.

Author: cpu

.github/workflows/ci.yml

@@ -56,7 +56,7 @@ jobs:
         with:
           toolchain: ${{ matrix.toolchain }}
       - name: cargo doc (all features)
-        run: cargo doc --all --all-features
+        run: cargo doc --all --all-features --document-private-items
         env:
           RUSTDOCFLAGS: '-Dwarnings'