MSVC: Document behavior when MSVC_RUNTIME_LIBRARY is not set

Brad Kingrequested to merge
brad.king/cmake:msvc-runtime-library-doc into master

By default CMAKE_MSVC_RUNTIME_LIBRARY is not set. Document the default runtime library selection in that case. We already test it.

Follow up to !3211 (merged) and !3249 (merged).

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