Add rustdoc to CI and fix some doc comments

This fixes a bunch of doc comments and also adds the run to CI so that
failures get caught early.

.github/workflows/ci.yml

@@ -40,6 +40,28 @@ jobs:
           token: ${{ secrets.GITHUB_TOKEN }}
           args: --workspace --tests --examples
 
+  docs:
+    name: Rust doc comments
+    runs-on: ubuntu-latest
+    env:
+      RUSTDOCFLAGS: -Dwarnings
+    steps:
+      - name: Checkout sources
+        uses: actions/checkout@v2
+      - name: Install rust stable toolchain
+        uses: actions-rs/toolchain@v1
+        with:
+          toolchain: stable
+          profile: minimal
+          components: rust-docs
+          override: true
+      - name: Cache rust cargo artifacts
+        uses: swatinem/rust-cache@v1
+      - name: Rustdoc
+        uses: actions-rs/cargo@v1
+        with:
+          command: doc
+          args: --workspace --document-private-items --no-deps
 
   build_and_test:
     name: Build and test