Hi,
I am a non expert kubuntu user and so i think that my problem
may be a no-problem.
Anyway your answer will be very interesting for me.
When I search the documentation about the collocation of files inherent
a kde application, I don't find it. Nothing on the KDE guide, nothing
on official kde internet site... sometime I find something
on the web page of a kde-user that worked to find something
on a specific subject.
When I search for some program or structure of linux I can start from 'man'
or 'info'. In this mode I find a lot of informations about the program.
For example
- the collocation of the files in the d.b. that is very important to use and
to configure the program,
-in which mode the program interacts with the o.s.
etc.
Unfortunately I am not able to do it with the kde applications and
from the kde documentation I almost learn to use the application
but with a insufficient knowledge.
A lot of things remains without answer.
For example to configure my lan on kubuntu 12.04 or 14.04
the graphic interface didn't work correctly and it
didn't cancel something. If I had this informations
I could do this manually e perhaps use a configuration
in the /etc/network/interfaces file...
I would like to know where I can find the informations
of kde programs and structures in the details such as
'man' and 'info' do (especially the collocation of the files
in the d.b. and in which mode the program interacts with the o.s.)
Thanks for your attention and Best Regards,
Paolo Gheri.

Well, first off Kubuntu != KDE. Kubuntu is a distribution bundling KDE's Plasma desktop and applications on top of an Ubuntu core. KDE is a collection of software, including the Plasma Desktop Environment and various (graphical) applications built on top of KDE framework libraries.

As for figuring out how to configure your LAN in Kubuntu, I'd suggest checking out the Kubuntu and Ubuntu forums where you'll get more specific recommendations to help resolve that.

For looking at documentation about KDE applications, most KDE apps will bring up a Help window with the F1 shortcut which will explain how to use and configure the application, explaining what all the different options are.
There is also the online UserBase wiki: https://userbase.kde.org/ where you may find more information.

For configuring KDE software, generally that is done using graphical tools. For desktop/environment settings, open up the System Settings tool, which is organized by category to help you find the specific bits you want to configure. In there there may be a module for Network Settings where you can configure some of the details of the network setting, though I'd guess that your problems deal with lower-level settings than are exposed in that module.
For application-specific settings, open up the application in question and go to Settings -> Configure <application> ... which will bring up the settings dialog for configuring that application.
Very few are the options not exposed via these settings dialogs.

If you do need to do direct manipulations on the config files, those are generally stored in ~/.config/ (KDE/Plasma 5, or ~/.kde/share/ in KDE/Plasma 4).
If you are interested in doing scripted config changes there are other threads on this forum which discuss how to do that.