Speed up CI runs by only cargo checking for docs

Running `cargo check --features testing_only_docs` will catch missing doc
comments, and we can avoid doing a full build.

Author: fitzgen

ci/assert-docs.sh

@@ -3,4 +3,4 @@
 set -xeu
 cd "$(dirname "$0")/.."
 
-cargo build --features "$BINDGEN_FEATURES testing_only_docs"
+cargo check --features "$BINDGEN_FEATURES testing_only_docs"