CJK is documented and the documentation is on CTAN. However, the documentation is in a compressed archive and is provided in the form of plain text files rather than DVI or PDF.
The archived documentation can be found here and includes files explaining how to use the package to typeset the different languages supported, together with the various options available.
cjk-ko provides enhanced support for Korean in the UTF8 encoding based on
At least on unix-like systems,
texdoc cjk gets you the basic documentation but
doc/latex/cjk/ provides further documentation and a directory of examples.