prepare_dives_for_divelogs(): remove unused variable 'doc'

Signed-off-by: Lubomir I. Ivanov <neolit123@gmail.com>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Usermanual final edit before Version 4.0

1) Many typos and lexical problems improved or corrected
2) Information for Heinrichs Weikamp DR5 moved to Appendix B
3) Small changes to images.

Signed-off-by: Willem Ferguson <willemferguson@zoology.up.ac.za>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>