[C++] Fix Doxygen error on `arrow::engine::ConversionStrictness`

Doxygen is hitting the following error: `/arrow/cpp/src/arrow/engine/substrait/options.h:37: error: documented symbol 'enum ARROW_ENGINE_EXPORT arrow::engine::arrow::engine::ConversionStrictness' was not declared or defined. (warning treated as error, aborting now)`. See this CI job output, for example.