ARROW-6677: [FlightRPC][C++] Document Flight in C++

[ghost]

• Adds StatusDetail to the docs
• Fixes the Doxygen config to work with ARROW_FLIGHT_EXPORT
• Adds error codes to the format docs (though they're not part of the formal format)
• Touches up some docstrings and adds missing classes to the API docs
• Add a basic description of how to set up a Flight server/client in C++

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@b780c56). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master    #5491   +/-   ##
=========================================
  Coverage          ?   89.23%           
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  Files             ?      768           
  Lines             ?   112435           
  Branches          ?        0           
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  Hits              ?   100328           
  Misses            ?    12107           
  Partials          ?        0

Impacted Files	Coverage Δ
cpp/src/arrow/status.h	93.54% <ø> (ø)
cpp/src/arrow/flight/types.h	100% <ø> (ø)
cpp/src/arrow/flight/server.h	100% <ø> (ø)

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[pitrou]

Rebased and fixed conflict.

[pitrou]

+1 from me, thank you!

[pitrou]

Triggered a build on my Travis-CI account: https://travis-ci.org/pitrou/arrow/builds/589335380

Will merge when green.

(edit: squashed to get full build)

[pitrou]

C++ and docs build green at https://ci.appveyor.com/project/pitrou/arrow/builds/27651743 and https://travis-ci.org/pitrou/arrow/builds/589335380