add more notes on changing epoch duration (#8491)

* add more notes on changing epoch duration

* add note about changing slot duration

substrate/bin/node-template/runtime/src/lib.rs

@@ -117,6 +117,8 @@ pub const VERSION: RuntimeVersion = RuntimeVersion {
 /// Change this to adjust the block time.
 pub const MILLISECS_PER_BLOCK: u64 = 6000;
 
+// NOTE: Currently it is not possible to change the slot duration after the chain has started.
+//       Attempting to do so will brick block production.
 pub const SLOT_DURATION: u64 = MILLISECS_PER_BLOCK;
 
 // Time is measured by number of blocks.