Make "acc" the default computation stage for sensors of type user.

PiperOrigin-RevId: 492689134
Change-Id: I21c07c74cb4b94d39c091ae98b169fd1235769d4

doc/XMLreference.rst

@@ -3943,7 +3943,7 @@ bodies whose center of mass is of interest.
    The type of output generated by this sensor. "axis" means a unit-length 3D vector. "quat" means a unit quaternion.
    These need to be declared because when MuJoCo adds noise, it must respect the vector normalization. "real" means a
    generic array (or scalar) of real values to which noise can be added independently.
-:at:`needstage`: :at-val:`[pos, vel, acc], required`
+:at:`needstage`: :at-val:`[pos, vel, acc], "acc"`
    The MuJoCo computation stage that must be completed before the user callback mjcb_sensor() is able to evaluate the
    output of this sensor.
 :at:`dim`: :at-val:`int, required`

doc/changelog.rst

@@ -69,8 +69,9 @@ General
   register one or more MuJoCo plugins on load.
 - Add an optional ``visualize`` callback to plugins, which is called during ``mjv_updateScene``. This callback allows
   custom plugin visualizations. Enable stree visualization for the Cable plugin as an example.
-- Sensors of type :ref:`user<sensor-user>` no longer require :at:`objtype` and :at:`objname`. If unspecified, the
-  objtype will be :ref:`mjOBJ_UNKNOWN<mjtObj>`. ``user`` sensors :at:`datatype` default is now :at-val:`"real"`.
+- Sensors of type :ref:`user<sensor-user>` no longer require :at:`objtype`, :at:`objname` and :at:`needstage`. If
+  unspecified, the objtype is now :ref:`mjOBJ_UNKNOWN<mjtObj>`. ``user`` sensors :at:`datatype` default is now 
+  :at-val:`"real"`, :at:`needstage` default is now :at-val:`"acc"`.
 - Add support for capsules in URDF import.
 - On macOS, issue an informative error message when run under `Rosetta 2 <https://support.apple.com/en-gb/HT211861>`_
   translation on an Apple Silicon machine. Pre-built MuJoCo binaries make use of