Use nightly toolchain in document CI workflow

.github/workflows/document.yml

@@ -10,9 +10,12 @@ jobs:
       group: ${{ github.workflow }}-${{ github.ref }}
     steps:
     - uses: hecrj/setup-rust-action@v1
+      with:
+        rust-version: nightly
     - uses: actions/checkout@v2
     - name: Generate documentation
       run: |
+        RUSTDOCFLAGS="--cfg docsrs" \
           cargo doc --no-deps --all-features \
           -p iced_core \
           -p iced_style \