I have installed python-doc. How can I access it?

Sorry if this is a such an obvious question, but where does Ubuntu put documentation packages for python, node.js, redis, mysql, mongodb, and other applications?

And why I cannot access it by typing python-doc in the terminal?

To examine help for all installed modules

:~#python
>>>help()
help>modules word_in_modulname
help>module_name
help>quit
>>>help('module_name')
>>>dir('module_or_class')

is how you do it in the python shell. One of those generates the output that shows you where to point a web browser for the docs.

pydoc -p portnumber

Might be what you were looking for. I'm assuming the answer wasn't sufficient yet.

If you want a gui, you might enjoy the package devhelp.

Since you have already installed python-doc, you can run dpkg --listfiles python-doc to get a list of all the files installed with that package. On my system, the place to start is /usr/share/doc/python-doc/html/index.html. This is simply a local copy of the documentation available online at http://docs.python.org. You can also download the documentation from http://docs.python.org/download.html, and save them in various formats anywhere you would like.

So, in general, doc packages go in /usr/share/doc. Typing python-doc in a terminal doesn't work because it is not the name of a command. The pydoc command can be useful, but a good option for you might be to make a link to the index.html file that came with the python-doc package, and put it somewhere convenient.