Trajectory: Improve docstrings

[v4hn]

Addresses https://github.com/ros-planning/moveit/pull/2751/files#r667319953

Please review @simonschmeisser

[simonschmeisser]

So this mainly affects what will be included when "exported" in a RobotTrajectoryMsg right? It seems not to be used in PlanningScene::isPathValid?

[codecov]

Codecov Report

Merging #2781 (d9094e9) into master (d1b4cd3) will increase coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2781      +/-   ##
==========================================
+ Coverage   60.15%   60.17%   +0.03%     
==========================================
  Files         356      356              
  Lines       31333    31333              
==========================================
+ Hits        18844    18852       +8     
+ Misses      12489    12481       -8     

Impacted Files	Coverage Δ
...include/moveit/robot_trajectory/robot_trajectory.h	98.00% <ø> (ø)
...on/pick_place/src/approach_and_translate_stage.cpp	73.38% <0.00%> (-0.64%)	⬇️
...dl_kinematics_plugin/src/kdl_kinematics_plugin.cpp	77.87% <0.00%> (-0.38%)	⬇️
...raint_samplers/src/default_constraint_samplers.cpp	80.72% <0.00%> (+0.30%)	⬆️
...meterization/work_space/pose_model_state_space.cpp	83.02% <0.00%> (+1.89%)	⬆️
.../ompl_interface/src/detail/constrained_sampler.cpp	59.46% <0.00%> (+16.22%)	⬆️

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[v4hn]

So this mainly affects what will be included when "exported" in a RobotTrajectoryMsg right? It seems not to be used in `PlanningScene::isPathValid`?

It's also used at least in the `unwind` methods. And should probably be used in others... And sadly you are right about isPathValid. :/ But I would say this is actually a shortcoming in the interface. Changing the semantics of the group argument default value `""` is probably a bad idea, but we could at least add another overload to the set that uses the group of the RobotTrajectory. Could you create a pull-request for that? ;) I intentionally kept the description here broad. Is that ok with you?

[simonschmeisser]

Ok, I hoped to get even more insight from more wordy doc-strings but since the "group" is not used too consistently I think this is as good as it gets