Update docs for the wasm executor crate (#3569)

* Grammar.

* Update docs for wasm executor.

* Update core/executor/src/lib.rs

Co-Authored-By: André Silva <andre.beat@gmail.com>

substrate/core/executor/src/lib.rs

@@ -14,16 +14,17 @@
 // You should have received a copy of the GNU General Public License
 // along with Substrate.  If not, see <http://www.gnu.org/licenses/>.
 
-//! Temporary crate for contracts implementations.
+//! A crate that provides means of executing/dispatching calls into the runtime.
 //!
-//! This will be replaced with WASM contracts stored on-chain.
-//! ** NOTE ***
-//! This is entirely deprecated with the idea of a single-module Wasm module for state transition.
-//! The dispatch table should be replaced with the specific functions needed:
-//! - execute_block(bytes)
-//! - init_block(PrevBlock?) -> InProgressBlock
-//! - add_transaction(InProgressBlock) -> InProgressBlock
-//! It is left as is for now as it might be removed before this is ever done.
+//! There are a few responsibilities of this crate at the moment:
+//!
+//! - It provides an implementation of a common entrypoint for calling into the runtime, both
+//! wasm and compiled.
+//! - It defines the environment for the wasm execution, namely the host functions that are to be
+//! provided into the wasm runtime module.
+//! - It also provides the required infrastructure for executing the current wasm runtime (specified
+//! by the current value of `:code` in the provided externalities), i.e. interfacing with
+//! wasm engine used, instance cache.
 
 #![warn(missing_docs)]
 #![recursion_limit="128"]