Fixing typos and syntax from User Manual

There was emphasising and bolding instructions in wrong order when
combined. At least on my system this must be started with *+ and ended
with +*. (There was also two cases when the ending was in same
order as starting also spoiling the XML syntax.) These prevented XML
validation thus terminating the compilation of the final documentation.
I fixed also a bunch of typos while at it.

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

Let's just give a try to opening ZIP files. Especially since we have one
of them in the test dives directory.

Signed-off-by: Miika Turkia <miika.turkia@gmail.com>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Skip tank volume if not present on UDCF import

Concatenating unit to empty value will give a "Strange volume reading"
error on subsurface (console output) when importing. Getting rid of it.

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

Pulled from Transifex, so just with my SOB.

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