mark GRANDPA and consensus stuff as unstable (#1469)

substrate/core/consensus/common/src/block_import.rs

@@ -137,8 +137,6 @@ impl<Block: BlockT> ImportBlock<Block> {
 	}
 }
 
-
-
 /// Block import trait.
 pub trait BlockImport<B: BlockT> {
 	type Error: ::std::error::Error + Send + 'static;

substrate/core/consensus/common/src/lib.rs

@@ -14,7 +14,11 @@
 // You should have received a copy of the GNU General Public License
 // along with Substrate Consensus Common.  If not, see <http://www.gnu.org/licenses/>.
 
-//! Tracks offline validators.
+//! Common utilities for building and using consensus engines in substrate.
+//!
+//! Much of this crate is _unstable_ and thus the API is likely to undergo
+//! change. Implementors of traits should not rely on the interfaces to remain
+//! the same.
 
 // This provides "unused" building blocks to other crates
 #![allow(dead_code)]