docs(system_diagnostic_graph): update readme for message format

[isamu-takagi]

Description

Update readme for message format.

Tests performed

Not applicable.

Effects on system behavior

Not applicable.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (765a596) 15.32% compared to head (d9896d2) 15.50%.
Report is 183 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5647      +/-   ##
==========================================
+ Coverage   15.32%   15.50%   +0.17%     
==========================================
  Files        1721     1725       +4     
  Lines      118559   118008     -551     
  Branches    37995    37807     -188     
==========================================
+ Hits        18169    18294     +125     
+ Misses      79657    79029     -628     
+ Partials    20733    20685      -48     

Flag	Coverage Δ		*Carryforward flag
differential	39.71% <ø> (?)
total	15.36% <ø> (+0.03%)	⬆️	Carriedforward from 2aa5d23

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

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[isamu-takagi]

Closed because it conflicts with #5827. The changes are included in that PR.