That's one of the beauties with FOSS. There isn't a central documentation for every piece of software (that is if the documentation exists in the first place).
Normally when looking for documentation, people rely on the
info and, if the application is a GUI, the yelp help files. That's normally with applications that users can interact with/configure/use directly (the frontend). This includes the
.conf files, and almost any text configuration file. Then some applications don't do this and rely on the
--help switch. Ubuntu (and Debian) has a policy for software documentation, that all documentation gets dumped in
ls -l /usr/share/doc/ | grep <package_name>
So, for the specific case of libraries, some of them includes
libetsf-io-doc - Developer documentation API and tutorials for ETSF_IO
Or in the specific case of your package
libaccounts-glib-doc - library for single signon - documentation
For finding this documentation you have to look for the source package
Then, there are some packages that don't include the documentation in the repositories for legal reasons, or just don't have any.
In any case, for finding any documentation, you should look for the
-doc of the packages, that normally are suggested (like
ntp-doc), or look for the content of a specific packages to see if it includes some info, man or README file:
dpkg -L <package> | grep -P '/man|info|README|doc/'