You didn’t tell any details on the system you are using so it’s hard to give you a proper advice. If you run openSUSE 42.1 with KDE as desktop you could go to
Yast2 -> Software Management -> View
Select “Pattern”.
Scroll down in the left part of the window and select “Documentation”.
That should give you a good deal (all?) of the available (depends on the repositories you have enabled) documentation packages.
As far back as I can remember, documentation integrated with GUI tools has been spotty at best, often broken.
Perhaps it’s a testament to how well various apps work that there hasn’t been a major outcry about inaccessible documentation (Believe me, it was sometimes greatly missed 10 years ago).
If you look in the documentation pattern, there are many types of documentation… MAN pages, components of the openSUSE base system, various components. In other words, there are many types of documentation which are stored in many parts of the system.
If you’re looking for book-like documentation, I’ve found them generally located as subdirectories of the following location
/usr/share/doc/
If you are looking for specific documentation and it’s not where you’d expect or you have no idea where to start, I’d recommend installing and using the locate utility (useful for finding any files or folders with lightning quickness)
To install and use locate,
Install
zypper in mlocate
After installation, its database will be updated daily but if you don’t want to wait, run the following immediately and any other time you don’t want to wait
updatedb
Then you can for instance list all available KDE documentation with the following
locate doc/kde
Documentation can be in many different formats, you might find html, pdf, other text. The docbook format in the above example should be automatically viewable by any KDE app when you click on the “Help” button, but you can also use a tool to convert to html or pdf if you want to read directly.