CmdStan User's guide - sampler auxiliary output file option `diagnostic-file` needs more doc

The CmdStan documentation should say a bit more about the output diagnostic-file option.
The sample methods use the output option diagnostic-file to designate files which contain the per-draw gradients of all parameters.
This is a .csv file, almost like the output file option's Stan CSV file (default name output.csv).

• Explain difference between CmdStan utility diagnose, Stan sampler method diagnose, and output diagnostics-file.

• Document the format of the output diagnostics-file - compare to output file=output.csv file:

  • Both files have config, header-rows, and timing.
  • The first columns of the data tables are the sampler state variables, lp__, accept_stat__, etc.
  • Only the output.csv file has adaptation information (as a comment following the data table header row)

Start in the CmdStan User's guide section covering sample output options, then compare the diagnostic-file output with the output.csv description. Document the distinction between CmdStan diagnose, the Stan sampler diagnose method, and diagnostic-file, including the shared configuration, headers, timing, and sampler-state columns and the adaptation information unique to output.csv.