[docs] Clarify coverage-dependent surface species docs

[ischoegl]

Changes proposed in this pull request

Clarify coverage-dependent surface species documentation

If applicable, fill in the issue number this pull request is fixing

Closes #1579

Checklist

• [x] The pull request includes a clear description of this code change
• [x] Commit messages have short titles and reference relevant issues
• [x] Build passes (scons build & scons test) and unit tests address code coverage
• [x] Style & formatting of contributed code follows contributing guidelines
• [x] The pull request is ready for review

[codecov[bot]]

Codecov Report

Merging #1583 (db1a5d4) into main (4b30fa7) will increase coverage by 0.07%. Report is 14 commits behind head on main. The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #1583      +/-   ##
==========================================
+ Coverage   70.51%   70.58%   +0.07%     
==========================================
  Files         379      379              
  Lines       59118    59134      +16     
  Branches    21237    21253      +16     
==========================================
+ Hits        41686    41739      +53     
+ Misses      14359    14322      -37     
  Partials     3073     3073              

see 8 files with indirect coverage changes

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[ischoegl]

Thanks for the feedback, @speth. This should be ready to merge.

[speth]

@ischoegl, please feel free to merge if this updated version makes sense to you.