[core] update some module docs

Author: Robbepop

core/src/env2/api.rs

@@ -1,4 +1,4 @@
-//! Provides the public raw interface towards the host Wasm engine.
+//! The public raw interface towards the host Wasm engine.
 //!
 //! # Note
 //!

core/src/env2/call/mod.rs

@@ -12,6 +12,8 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+//! Utilities to call or instantiate contracts on the chain.
+
 mod builder;
 mod create;
 mod utils;