docs: document async_db_url and reflex[db] extra (ENG-8371)

docs/database/overview.md

@@ -18,6 +18,15 @@ For advanced use cases, please see the
 If you are using a NoSQL database (e.g. MongoDB), you can work with it in Reflex by installing the appropriate Python client library. In this case, Reflex will not provide any ORM features.
 ```
 
+## Installation
+
+The ORM dependencies (SQLModel and Alembic) are an optional extra.
+Install them with the `db` extra:
+
+```bash
+pip install "reflex[db]"
+```
+
 ## Connecting
 
 Reflex provides a built-in SQLite database for storing and retrieving data.

docs/database/queries.md

@@ -194,6 +194,35 @@ class State(rx.State):
 
 Reflex provides an async version of the session function called `rx.asession` for asynchronous database operations. This is useful when you need to perform database operations in an async context, such as within async event handlers.
 
+### Configuring the Async Database URL
+
+`rx.asession` needs its own `async_db_url` in `rxconfig.py`. It must point at
+the same database as `db_url` but use an async driver. Calling `rx.asession()`
+when it is unset raises an error.
+
+```python
+config = rx.Config(
+    app_name="my_app",
+    db_url="sqlite:///reflex.db",
+    async_db_url="sqlite+aiosqlite:///reflex.db",
+)
+```
+
+The matching async DBAPI driver is not part of the `reflex[db]` extra, so
+install it separately. For the SQLite URL above, install `aiosqlite`:
+
+```bash
+pip install aiosqlite
+```
+
+For PostgreSQL, install `psycopg` (psycopg3). It works as both the sync and
+async driver, so `db_url` and `async_db_url` can share one
+`postgresql+psycopg://...` scheme:
+
+```bash
+pip install "psycopg[binary]"
+```
+
 The `rx.asession` function returns an async SQLAlchemy session that must be used with an async context manager. Most operations against the `asession` must be awaited.
 
 ```python