Here's an example: I was looking for the document, "Compilation and Analysis of Legal Norms," which carries the symbol: E/CN.4/1996/52/Add.2.
As per the instructions above, I created the following new URL -- http://undocs.org/E/CN.4/1996/52/Add.2 -- which I then entered into my web browser, and voila! Note that the language version of the document defaults to English, but other available language options are displayed in a header running across the top of the screen.
Ultimately, this search serves as a short-cut way to locate documents housed in the UN's Official Document System (ODS). Which means that if a document is not actually available in the ODS, the strategy won't work. This may be because a document is older and has not yet been scanned. Another time I came up empty-handed is when I searched for http://undocs.org/A/69/CRP.1, which happens to be a very recent document: the Recommended Principles and Guidelines on Human Rights at International Borders. So perhaps Conference Room Papers are not included in the ODS either.
The only way to find out what you can access is to give it a go!
[Image source: UNRIC Library Newsletter, June 2011]