MSVC: Document behavior when MSVC_RUNTIME_LIBRARY is not set
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).
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).