Mocap bodies are the weld root of their own kinematic subtree.

PiperOrigin-RevId: 960212286
Change-Id: Ibeff129c3110576c8846c76ba6ab756f23c0fa2e

doc/overview.rst

@@ -804,13 +804,17 @@ section, prevents objects from falling through the floor or moving through walls
 the following situation:
 
 The user comments out the root joint of a floating-base model, perhaps in order to prevent it from falling; now that the
-base body is counted as static, new collisions appear that were not there before and the user is confused. There are two
-easy ways to avoid this problem:
+base body is counted as static, new collisions appear that were not there before and the user is confused. There are
+three easy ways to avoid this problem:
 
-1. Don't remove the root joint. Perhaps it is enough to :ref:`disable gravity<option-flag>` and possibly add some
+1. Make the newly static body a :ref:`mocap body<body-mocap>`. Mocap bodies are the root of their own body group for
+   the purposes of collision filtering, rather than being welded to the world, so the parent-child exclusion applies to
+   their children as usual. As a bonus, the body can be moved around interactively.
+
+2. Don't remove the root joint. Perhaps it is enough to :ref:`disable gravity<option-flag>` and possibly add some
    :ref:`fluid viscosity<option>` in order to prevent your model from moving around too much.
 
-2. Use :ref:`collision filtering<Collision>` to explicitly disable the unwanted collisions, either by setting the
+3. Use :ref:`collision filtering<Collision>` to explicitly disable the unwanted collisions, either by setting the
    relevant :at:`contype` and :at:`conaffinity` attributes, or by using a contact :ref:`exclude <contact-exclude>`
    directive.