Improve mongoexport help output for csv

Using the --csv option requires that fields be specified using -f. This makes sense and if you don't specify fields mongoexport spits out a short error message:

./mongoexport -d bi -c game_event -o st_action.csv --csv
connected to: 127.0.0.1
assertion: 9998 you need to specify fields

However, the help output doesn't currently mention that -csv requires -f:

-f [ --fields ] arg comma separated list of field names e.g. -f
name,age
--csv export to csv instead of json

The help output should mention that you have to use -f with --csv and explain why that is the case.