Update doxygen comments for distance() and interpolate()

[AndyZe]

Update documentation of interpolate() and distance() to clarify a few things:

• The t parameter of interpolate isn't a time, it's a fraction
• distance() just sums the joint difference, for all joints. It's not an L2 norm or SSE or anything like that. Verify here.

Fixes #2527

[tylerjw]

Thank you for doing this! I read through these and they look good. I'll merge this once it passes CI if no one else requests changes.

[codecov]

Codecov Report

Merging #2528 (a62bb4d) into master (41771d0) will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2528      +/-   ##
==========================================
- Coverage   60.23%   60.22%   -0.00%     
==========================================
  Files         351      351              
  Lines       26470    26470              
==========================================
- Hits        15942    15940       -2     
- Misses      10528    10530       +2     

Impacted Files	Coverage Δ
...bot_model/include/moveit/robot_model/robot_model.h	84.22% <ø> (ø)
...bot_state/include/moveit/robot_state/robot_state.h	91.12% <ø> (ø)
...nning_scene_monitor/src/planning_scene_monitor.cpp	67.84% <0.00%> (-0.31%)	⬇️

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[simonschmeisser]

thanks especially for the corner-case info about active joints!

[v4hn]

`distance()` just sums the joint difference, for all joints. It's not an L2 norm or SSE or anything like that.

Just because of the way you formulate this: summing up element-wise differences is the standard [L1 norm](https://en.wikipedia.org/wiki/Taxicab_geometry) and I'm missing that term a bit in your patch. There was even a proposal to add weighting to the components, but it was not merged as far as I know.

[AndyZe]

distance() just sums the joint difference, for all joints. It's not an L2 norm or SSE or anything like that.
Just because of the way you formulate this: summing up element-wise differences is the standard L1 norm and I'm missing that term a bit in your patch. There was even a proposal to add weighting to the components, but it was not merged as far as I know.

I'm porting this to MoveIt2 now and will mention L1 norm 👍