models/contest: clarify rate_all behaviour

[Riolku]

Rate all is confusingly named, since it really means "rate even users who don't submit". In my opinion, this should have been the default, but instead we can simply fix the description.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 46.73%. Comparing base (fd7fb05) to head (c181ab9). Report is 17 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2347      +/-   ##
==========================================
- Coverage   46.76%   46.73%   -0.03%     
==========================================
  Files         251      254       +3     
  Lines       13317    13394      +77     
==========================================
+ Hits         6228     6260      +32     
- Misses       7089     7134      +45     

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