update docs on activation_height() method

zebra-chain/src/parameters/network_upgrade.rs

@@ -326,10 +326,13 @@ impl NetworkUpgrade {
     /// such as on Regtest or a configured Testnet where multiple network upgrades have the
     /// same activation height, or if one is omitted when others that follow it are included.
     ///
-    /// Returns None if this network upgrade is a future upgrade, and its
-    /// activation height has not been set yet.
-    ///
-    /// Returns None if this network upgrade has not been configured on a Testnet or Regtest.
+    /// Returns [`Height::MAX`] if no activation height is set for this network upgrade or any
+    /// future network upgrade.
+    // TODO:
+    // Returns None if this network upgrade is a future upgrade, and its
+    // activation height has not been set yet.
+    //
+    // Returns None if this network upgrade has not been configured on a Testnet or Regtest.
     pub fn activation_height(&self, network: &Network) -> Option<block::Height> {
         network
             .activation_list()