MastForest::add_node(): add docs

core/src/mast/mod.rs

@@ -60,6 +60,8 @@ impl MastForest {
 /// Mutators
 impl MastForest {
     /// Adds a node to the forest, and returns the associated [`MastNodeId`].
+    /// 
+    /// Adding two duplicate nodes will result in two distinct returned [`MastNodeId`]s.
     pub fn add_node(&mut self, node: MastNode) -> MastNodeId {
         let new_node_id = MastNodeId(
             self.nodes