docs: update for venv_backend

Signed-off-by: Henry Schreiner <[email protected]>

docs/config.rst

@@ -167,14 +167,19 @@ You can also specify that the virtualenv should *always* be reused instead of re
     def tests(session):
         pass
 
-You are not limited to virtualenv, there is a selection of backends you can choose from as venv, conda, mamba, or virtualenv (default):
+You are not limited to virtualenv, there is a selection of backends you can choose from as venv, uv, conda, mamba, or virtualenv (default):
 
 .. code-block:: python
 
     @nox.session(venv_backend='venv')
     def tests(session):
         pass
 
+You can chain together optional backends with ``|``, such as ``uv|virtualenv``
+or ``mamba|conda``, and the first available backend will be selected. You
+cannot put anything after a backend that can't be missing like ``venv`` or
+``virtualenv``.
+
 Finally, custom backend parameters are supported:
 
 .. code-block:: python
@@ -183,6 +188,9 @@ Finally, custom backend parameters are supported:
     def tests(session):
         pass
 
+If you need to check to see which backend was selected, you can access it via
+``session.venv_backend``.
+
 
 Passing arguments into sessions
 -------------------------------

docs/usage.rst

@@ -165,6 +165,9 @@ Note that using this option does not change the backend for sessions where ``ven
 
 Backends that could be missing (``uv``, ``conda``, and ``mamba``) can have a fallback using ``|``, such as ``uv|virtualenv`` or ``mamba|conda``. This will use the first item that is available on the users system.
 
+If you need to check to see which backend was selected, you can access it via
+``session.venv_backend`` in your noxfile.
+
 .. _opt-force-venv-backend:
 
 Forcing the sessions backend