Add mjENBL_DIAGEXACT for exact constraint diagonal. Fixes #2472

PiperOrigin-RevId: 916932908
Change-Id: Id23ac39b5cd996afc52990719a4e07c0cc7de600

doc/modeling.rst

@@ -1855,6 +1855,25 @@ in a visible way, and the energy fluctuates around the initial value instead of
    </worldbody>
 
 
+.. _CConstraintImpedance:
+
+Constraint accuracy
+~~~~~~~~~~~~~~~~~~~
+MuJoCo's :ref:`constraint impedance<soParameters>` computation relies on an approximate diagonal of the constraint-space
+inertia matrix, computed once at compile time from the initial configuration ``qpos0``.
+
+In the vast majority of models this approximation is entirely adequate. However, in certain situations—such as models
+with highly anisotropic inertias, complex kinematic chains, or bodies operating far from ``qpos0``—the approximation
+may become inaccurate. This can occasionally manifest as unexplained solver divergence (``badqacc`` warnings),
+excessive penetration, unrealistic slip, or poor solver convergence. A useful diagnostic is the
+:ref:`fwdinv<option-flag-fwdinv>` flag: if the forward-inverse discrepancy is large, inaccurate constraint scaling may
+be a contributing factor.
+
+If you suspect that the compile-time approximation is insufficient for your model, you can enable the
+:ref:`diagexact<option-flag-diagexact>` flag to compute the exact diagonal at runtime. See :ref:`Diagonal approximation
+<soExactDiag>` for details on the underlying mechanics and performance implications.
+
+
 .. |image3| image:: images/modeling/tendonwraps.png
    :width: 500px
 .. |image4| image:: images/modeling/particle.png