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ARROW-5088: [C++] Only add -Werror in debug builds. Add C++ documentation about compiler warning levels

I think there are two reasonable options:

* Only add `-Werror` in debug builds
* Disallow use of BUILD_WARNING_LEVEL=CHECKIN in release builds altogether

I think it's useful at least to be able to _see_ the warnings, so I went with the first option.

Author: Wes McKinney <wesm+git@apache.org>

Closes #4093 from wesm/ARROW-5088 and squashes the following commits:

7689a1483 <Wes McKinney> Grammar
15fc98efc <Wes McKinney> Only add -Werror in debug builds. Add C++ documentation about compiler warning levels