Make mapping for YcmShowDetailedDiagnostic <silent>

[Aster89]

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Why this change is necessary and useful

This change is not necessary, but it's moderately useful in that it avoids the unnecessary visual noise of echoing :YcmShowDetailedDiagnostic on the command line when triggering the detailed diagnostic popup, in the case that one has the following setting in place

let g:ycm_show_detailed_diag_in_popup = 1

Rationale for not having a test

I think the change is uncontroversially useful, despite of very mild usefulness, and fundamentally safe. I don't think any existing reliance on this command not being <silent> would be justifiable in the first place.

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This change is 

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.67%. Comparing base (6a52780) to head (cbd4ceb).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4333      +/-   ##
==========================================
+ Coverage   89.61%   89.67%   +0.06%     
==========================================
  Files          37       37              
  Lines        4765     4765              
==========================================
+ Hits         4270     4273       +3     
+ Misses        495      492       -3     

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