Add a few words about transactions using the reactive SQL clients

docs/src/main/asciidoc/reactive-sql-clients.adoc

@@ -488,6 +488,58 @@ Navigate to http://localhost:8080/fruits.html and read/create/delete some fruits
 |`io.vertx.mutiny.mysqlclient.MySQLPool`
 |===
 
+== Transactions
+
+The reactive SQL clients support transactions.
+A transaction is started with `client.begin()` and terminated with either `tx.commit()` or `tx.rollback()`.
+All these operations are asynchronous:
+
+* `client.begin()` returns a `Uni<Transaction>`
+* `client.commit()` and `client.rollback()` return `Uni<Void>`
+
+The following snippet shows how to run 2 insertions in the same transaction:
+
+[source, java]
+----
+public static Uni<Void> insertTwoFruits(PgPool client, Fruit fruit1, Fruit fruit2) {
+    return client.begin()
+            .flatMap(tx -> {
+                Uni<RowSet<Row>> insertOne = tx.preparedQuery("INSERT INTO fruits (name) VALUES ($1) RETURNING (id)")
+                        .execute(Tuple.of(fruit1.name));
+                Uni<RowSet<Row>> insertTwo = tx.preparedQuery("INSERT INTO fruits (name) VALUES ($1) RETURNING (id)")
+                        .execute(Tuple.of(fruit2.name));
+
+                return insertOne.and(insertTwo)
+                        // Ignore the results (the two ids)
+                        .onItem().ignore().andContinueWithNull()
+                        // on success, commit
+                        .onItem().produceUni(x -> tx.commit())
+                        // on failure rollback
+                        .onFailure().recoverWithUni(tx::rollback);
+            });
+}
+----
+
+In this example, the two insertions are not dependant on each other and are executed concurrently (but in the same transaction).
+This transaction is committed on success and rolled back on failure.
+
+You can also create dependant actions as follows:
+
+[source, java]
+----
+return this.client.begin()
+       .flatMap(tx -> tx
+               .preparedQuery("INSERT INTO person (firstname,lastname) VALUES ($1,$2) RETURNING (id)",
+                       Tuple.of(person.getFirstName(),person.getLastName()))
+
+               .onItem().produceUni(id-> tx.preparedQuery("INSERT INTO addr (person_id,addrline1) VALUES ($1,$2)",
+                       Tuple.of(id.iterator().next().getLong("id"),person.getLastName())))
+
+               .onItem().produceUni(res -> tx.commit())
+               .onFailure().recoverWithUni(ex-> tx.rollback())
+       );
+----
+
 == Configuration Reference
 
 === Common Datasource