Auto merge of #825 - fitzgen:ci-assert-dosc, r=emilio

Speed up CI runs by only `cargo check`ing for docs

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

r? @emilio

Author: bors-servo

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"