Add export CSV to user manual

Mention the option to export in CSV format in our user manual.

Signed-off-by: Miika Turkia <miika.turkia@gmail.com>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Improve header wording for CSV export

Duration is better term than time for the length of the dive. Also
remove the repetitive word dive from some of the header fields.

Signed-off-by: Miika Turkia <miika.turkia@gmail.com>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>