SNOW-1636484 document certificate checking in .NET driver

[sfc-gh-dszmolka]

Description

This is a documentation-only change, describing how certificate validation works today in .NET driver as opposed to all the other Snowflake drivers, and describing some possible gotchas to pay attention for.

Checklist

• Code compiles correctly
• Code is formatted according to Coding Conventions
• Created tests which fail without the change (if possible)
• All tests passing (dotnet test)
• Extended the README / documentation, if necessary
• Provide JIRA issue id (if possible) or GitHub issue id in PR name

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.66%. Comparing base (c3cb344) to head (738741e).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1015      +/-   ##
==========================================
- Coverage   85.70%   85.66%   -0.04%     
==========================================
  Files         117      117              
  Lines       11331    11331              
  Branches     1130     1130              
==========================================
- Hits         9711     9707       -4     
- Misses       1342     1345       +3     
- Partials      278      279       +1     

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