Change finite-support mju_sigmoid from a two-quadratic construction to a single quintic with zero 1st and 2nd derivatives at 0 and 1, the boundary of the smooth-support segment.

Note: this function is currently unused in the MuJoCo codebase.
PiperOrigin-RevId: 529969804
Change-Id: I2bddc72916eff67625b806510935bc7f3ce5c770

introspect/functions.py

@@ -7752,7 +7752,7 @@ FUNCTIONS: Mapping[str, FunctionDecl] = dict([
                  type=ValueType(name='mjtNum'),
              ),
          ),
-         doc='Sigmoid function over 0<=x<=1 constructed from half-quadratics.',
+         doc='Sigmoid function over 0<=x<=1 using quintic polynomial.',
      )),
     ('mjd_transitionFD',
      FunctionDecl(