Add light radius attribute.

Also fix bug: light defaults were not being saved.

PiperOrigin-RevId: 625142278
Change-Id: I8ec3a5a06e9f314e98b7c34346db3c24b34d7e0b

doc/XMLreference.rst

@@ -3181,6 +3181,12 @@ the direction specified by the dir attribute. It does not have a full spatial fr
 :at:`dir`: :at-val:`real(3), "0 0 -1"`
    Direction of the light.
 
+.. _body-light-radius:
+
+:at:`radius`: :at-val:`real, "0"`
+   Radius of the light, affects shadow softness. This attribute has no effect in MuJoCo's native renderer, but it can be
+   useful when rendering scenes with an external renderer.
+
 .. _body-light-attenuation:
 
 :at:`attenuation`: :at-val:`real(3), "1 0 0"`
@@ -7586,6 +7592,8 @@ if omitted.
 
 .. _default-light-dir:
 
+.. _default-light-radius:
+
 .. _default-light-directional:
 
 .. _default-light-castshadow: