polish data model doc

[innerpeacez]

• If this pull request closes/resolves/fixes an existing issue, replace the issue number. Closes https://github.com/apache/skywalking/issues/.
• Update the CHANGES log.

[codecov-commenter]

Codecov Report

Merging #291 (156ad7b) into main (1f2e8f4) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #291   +/-   ##
=======================================
  Coverage   44.20%   44.20%           
=======================================
  Files          93       93           
  Lines        9804     9804           
=======================================
  Hits         4334     4334           
  Misses       5052     5052           
  Partials      418      418           

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