docs(mongo-adapter): clarify transaction setup (#11550)

docs/content/docs/adapters/mongo.mdx

@@ -19,24 +19,44 @@ To use the MongoDB adapter, you need to install the `@better-auth/mongo-adapter`
 
 ## Example Usage
 
-You can use the MongoDB adapter to connect to your database as follows.
+### Replica set or sharded cluster
+
+Pass `client` so the adapter uses transactions by default. Use a connection string for your replica set or sharded cluster:
 
 ```ts title="auth.ts"
 import { betterAuth } from "better-auth";
 import { MongoClient } from "mongodb";
-import { mongodbAdapter } from "better-auth/adapters/mongodb";
+import { mongodbAdapter } from "@better-auth/mongo-adapter";
 
 const client = new MongoClient("mongodb://localhost:27017/database");
 const db = client.db();
 
 export const auth = betterAuth({
   database: mongodbAdapter(db, {
-    // Optional: if you don't provide a client, database transactions won't be enabled.
-    client
+    client,
   }),
 });
 ```
 
+### Standalone server
+
+A standalone MongoDB server does not support transactions. Create `db` with `MongoClient` and pass it to the adapter without the `client` option:
+
+```ts title="auth.ts"
+import { betterAuth } from "better-auth";
+import { MongoClient } from "mongodb";
+import { mongodbAdapter } from "@better-auth/mongo-adapter";
+
+const client = new MongoClient("mongodb://localhost:27017/database");
+const db = client.db();
+
+export const auth = betterAuth({
+  database: mongodbAdapter(db),
+});
+```
+
+Without transactions, an earlier write may remain if a later write fails. When you move to a replica set or sharded cluster, pass `client` to enable transactions. See [MongoDB's transaction requirements](https://www.mongodb.com/docs/manual/core/transactions-production-consideration/#availability).
+
 ## Schema generation & migration
 
 For MongoDB, we don't need to generate or migrate the schema.

docs/content/docs/installation.mdx

@@ -157,9 +157,12 @@ description: Learn how to configure Better Auth in your project.
         import { client } from "@/db"; // your mongodb client
 
         export const auth = betterAuth({
-            database: mongodbAdapter(client),
+            database: mongodbAdapter(client.db(), { client }),
         });
         ```
+
+        See the [MongoDB adapter guide](/docs/adapters/mongo) for standalone
+        server setup.
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