file_reporter argument to RunSpecifiedBenchmarks should be optional, but is not

In the comment/documentation of `RunSpecifiedBenchmarks` in benchmark.h, it says:

> // The second and third overload use the specified 'display_reporter' and
> // 'file_reporter' respectively. 'file_reporter' will write to the file
> // specified
> // by '--benchmark_output'. If '--benchmark_output' is not given the
> // 'file_reporter' is ignored.

The first problem is that the correct argument name is benchmark_out (I assume it was changed at some point).
The second problem is that the last sentence is not true. If ' --benchmark_out' is not given, the following message is displayed:

> A custom file reporter was provided but --benchmark_out= was not specified.

I don't know which is the intended behavior, but to me, it would make sense that the `file_reporter` is ignored if '--benchmark_out' is not specified.