Updates to the user-manual: DivePlanning

- Added some more text to Miika's great start describing the planner.
- Tried doing something more with the formatting options asciidoc give us.

[Dirk Hohndel: merged with my earlier changes to the same section]

Signed-off-by: Reinout Hoornweg <reinout@xs4all.nl>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>