docs(goal_planner): modify threads docs

[kosuke55]

Description

modify threads docs

Related links

Tests performed

none

Notes for reviewers

Interface changes

none

Effects on system behavior

none

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In-review checklist for the PR reviewers

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[kyoichi-sugahara]

LGTM!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (64799b6) 14.38% compared to head (ae1be9d) 14.38%.
Report is 10 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6295   +/-   ##
=======================================
  Coverage   14.38%   14.38%           
=======================================
  Files        1907     1907           
  Lines      129929   129929           
  Branches    37613    37613           
=======================================
  Hits        18695    18695           
  Misses      90224    90224           
  Partials    21010    21010           

Flag	Coverage Δ		*Carryforward flag
differential	0.00% <ø> (?)
total	14.38% <ø> (ø)		Carriedforward from 64799b6

*This pull request uses carry forward flags. Click here to find out more.

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