Documentation of time-dependent output out of date

[mrhardman]

The documentation for the writing of output as a function of time appears to be out of date.

The example code shown in the documentation https://bout-dev.readthedocs.io/en/stable/user_docs/bout_options.html#time-dependence does not compile, but will compile if the final line is modified to

    // Append data to file
    bout::OptionsIO::create({{"file", "time.nc"}, {"append", true}})->write(data);

Including information in the documentation explaining how the output interfaces with MPI would be useful. I.e., explaining whether or not these calls should be made collectively or on a single process.