[PATCH] A few tiny changes to user-manual.txt

[Miika Turkia]

If we have someone anxious to do some work on the documentation. I
think the importing from different software could use some loving. I
think one chapter for the import is enough and each different
application import should describe only the steps to produce usable
output from the app in question. (Currently e.g. chapters Importing
from JDiveLog, MacDive and divelogs.de all describe how to import to
Subsurface, even though this part is the same for all programs.)

miika