Update documentation for g:ycm_completer_settings

[Felixoid]

PR Prelude

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Why this change is necessary and useful

This is a follow-up to ycm-core/ycmd#1813 in case it is merged.

I hope the g:ycm_completer_settings eases the user's life, and I have prepared documentation in advance.

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This change is 

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.73%. Comparing base (6a52780) to head (c1bea5d).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4332      +/-   ##
==========================================
+ Coverage   89.61%   89.73%   +0.12%     
==========================================
  Files          37       37              
  Lines        4765     4765              
==========================================
+ Hits         4270     4276       +6     
+ Misses        495      489       -6     

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