Clarify `jointParents` value for root bones in `XR_FB_hand_tracking_mesh`

I'm adding support for the `XR_FB_hand_tracking_mesh` extension to my application, and it wasn't clear to me what the `jointParents` value of the `XrHandTrackingMeshFB` struct would be for root bones. Based on my testing it appears the joint parent value for root bones is `XR_HAND_JOINT_MAX_ENUM_EXT`. I think it would be useful to clarify that in the specification.

Locate the XR_FB_hand_tracking_mesh specification text describing the XrHandTrackingMeshFB jointParents member and read how root bones are defined. Clarify that root bones use XR_HAND_JOINT_MAX_ENUM_EXT, if the specification confirms the reported behavior, and verify the wording is consistent with the surrounding extension text.

Written by the indexing model from the issue text.