Silly change to a comment

I had intended to amend the previous commit with this before pushing it
out. This changes the comment that pointed out the bug that the previous
commit fixed.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Be more careful about dive computer selection

The selection logic was a bit random: some places would return NULL if
the dive computer index was out of range, others would return the
primary dive computer, and actually moving between dive computers would
just blindly increment and decrement the number.

This always selects the primary computer if the index is out of bounds,
and makes sure we stay in bound when switching beteen dive computers
(but switching between dives can then turn an in-bound number into an
out-of-bounds one)

Fixes #464

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
User manual: Changes to sections 1 and 2

The text for these two sections has been corrected and updated
in order to be ready for the release of V4.1. The very first
paragraph of the manual is expanded substantially.

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