Update comments per review

Indicate that Model::Load can also call private method.
State that only relative names are supported in graphs in
FrameSemantics.hh

Signed-off-by: Steve Peters <scpeters@openrobotics.org>

include/sdf/Model.hh

@@ -304,7 +304,7 @@ namespace sdf
 
     /// \brief Give a weak pointer to the PoseRelativeToGraph to be used
     /// for resolving poses. This is private and is intended to be called by
-    /// World::Load.
+    /// World::Load and Model::Load if this is a nested model.
     /// \param[in] _graph Weak pointer to PoseRelativeToGraph.
     /// \return Error if graph pointer is invalid.
     private: sdf::Errors SetPoseRelativeToGraph(

src/FrameSemantics.hh

@@ -32,6 +32,11 @@
 /// The Frame Semantics Utilities construct and operate on graphs representing
 /// the kinematics, frame attached_to, and pose relative_to relationships
 /// defined within models and world.
+///
+/// Note that all graphs should only contain relative names (e.g. "my_link"),
+/// not absolute names ("top_model::nested_model::my_link").
+/// Graphs inside nested models (currently via directly nested models in
+/// //model or //world elements) will be explicitly separate graphs.
 namespace sdf
 {
   // Inline bracket to help doxygen filtering.