Clarify the semantics of if timestamps in nav_msgs/Path

This is a follwup to the pre Foxy Message API review

Path message is stamped, but each individual pose is also stamped. It's unclear what each stamp means. Is the stamp in each individual pose for the time the robot needs to be at that location? If so, the frame_id that's in each individual header is still redundant with the frame_id on the Path message itself. However, stamping each individual pose seems inflexible to the robot being delayed. If it's slowed down mid path, should it target the pose at the current time, or the next closest to it's current location? This seems hard to answer if the path makes a loop such that two points on the path are physically close but far apart in time. For a little while nav2 had it's own version of Path which had non-stamped poses, though they removed it: https://github.com/ros-planning/navigation2/pull/1107/files#diff-2d8dabb75c11aa980f6c2629eba1e75f

Iterate with navigation to improve documentation of semantics of timestamps. Note that there are multiple valid behaviors for any given path representation.

Start with the nav_msgs/Path message definition and the linked pre-Foxy Message API review. Review the timestamp and frame_id discussion with navigation stakeholders; done means the agreed semantics and valid path behaviors are documented clearly.