User manual: some minor readability and repeats locationless GPS warn

Some minor readability tweaks.
Repeats wanrning about co-ordinates without Location name, all the
other info on co-ordinates was cut-pasted from the manual dive
entry section, but this bit was missing.

Signed-off-by: Tim Wootton <tim@tee-jay.demon.co.uk>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
User manual: make field edit decriptions consistent

Make descriptions of field editing consistent between the manually
added and DC imported sections. I preferred the DC section wording
on ratings.

Signed-off-by: Tim Wootton <tim@tee-jay.demon.co.uk>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
User manual: use synchronisation as we do elsewhere and into not onto

Even though its Synchronization in the menu (in the base language
anyway) elsewhere thoughout the manual we use the en_GB spelling.
Use "into" rather than "onto" subsurface

Signed-off-by: Tim Wootton <tim@tee-jay.demon.co.uk>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
User manual: remove avg. depth. relocate picture ref and minor corrections

Multi-dive stats no longer include avg. depth
Picture was above, not below
Several minor corrections

Signed-off-by: Tim Wootton <tim@tee-jay.demon.co.uk>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Maintab: fix trip location / notes editing

Reusing the displayed_dive for this caused all kind of odd problems that
were hard to reproduce, because the behavior depended on what was in the
corresponding fields of the current_dive. Worse: the GPS location handling
prevented us from reliably removing the location of a trip.

The solution isn't ideal and certainly isn't elegant. Maybe we simply
shouldn't reuse the widget here. But I think what I have now works - I
tried hard to make it break again and couldn't.

Fixes #659

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>