The word description is overloaded in `sdf::Element`

The word `description` in `sdf::Element::PrintDescription` or `sdf::Element::AddElementDescription` has a different meaning than in `sdf::Element::GetDescription`. The former refers to an XML document that describes what the valid elements are in the given `sdf::Element` object. Another name for this could be "schema". The latter refers to the string in the `` tag of the schema.

For example: the description as in the schema for the `` tag is https://github.com/gazebosim/sdformat/blob/da1f325b06f1c1bdd9bced0f67e87df871d0ed80/sdf/1.10/pose.sdf#L1-L43

But the description as in string in ``
> A pose (translation, rotation) expressed in the frame named by
@relative_to. The first three components (x, y, z) represent the position of
the element's origin (in the @relative_to frame). The rotation component
represents the orientation of the element as either a sequence of Euler
rotations (r, p, y), see http://sdformat.org/tutorials?tut=specify_pose,
or as a quaternion (x, y, z, w), where w is the real component.

I propose we change our naming for the first case to "schema".