diff --git a/src/transaction.rs b/src/transaction.rs
index 41414f7..7e3e012 100644
--- a/src/transaction.rs
+++ b/src/transaction.rs
@@ -94,6 +94,9 @@ pub struct Savepoint<'conn> {
 
 impl<'conn> Transaction<'conn> {
     /// Begin a new transaction. Cannot be nested; see `savepoint` for nested transactions.
+    // Even though we don't mutate the connection, we take a `&mut Connection`
+    // so as to prevent nested or concurrent transactions on the same
+    // connection.
     pub fn new(conn: &mut Connection, behavior: TransactionBehavior) -> Result<Transaction> {
         let query = match behavior {
             TransactionBehavior::Deferred => "BEGIN DEFERRED",