Add description of new experimental interops

doc/interop-test-descriptions.md

@@ -899,6 +899,25 @@ Status: TODO
 This test verifies that a client sending faster than a server can drain sees
 pushback (i.e., attempts to send succeed only after appropriate delays).
 
+### Experimental Tests
+
+These tests are not yet standardized, and are not yet implemented in all
+languages. Therefore they are not part of our interop matrix.
+
+#### rpc_soak
+
+The client performs many large_unary RPCs in sequence over the same channel. 
+The number of RPCs is configured by the experimental flag, `soak_iterations`.
+
+#### channel_soak
+
+The client performs many large_unary RPCs in sequence. Before each RPC, it 
+tears down and rebuilds the channel. The number of RPCs is configured by 
+the experimental flag, `soak_iterations`.
+
+This tests puts stress on several gRPC components; the resolver, the load 
+balancer, and the RPC hotpath.
+
 ### TODO Tests
 
 #### High priority: