Since the documentation on SDL Tridion World (http://www.sdltridionworld.com) and the SDL WCMS Documentation portal (also known as SDL Live Content, http://sdllivecontent.sdl.com/LiveContent) are only available through a login, I usually refer to them with the comment (in parentheses) that a login is required.
The link I use for the login is http://docportal.sdl.com/sdltridion, this is the same link which is used in the SDL Tridion Dashboard page under Help & Community and it currently redirects to a page on SDL Tridion World indicating how you can obtain the login details for SDL Tridion World and where you can find the Documentation.
In the future the http://docportal.sdl.com/sdltridion link might be redirected to a different location (an SDL wide documentation portal), so it is a safe link to use.
My suggestion indeed is to always mention when a certain link requires a login which can be done using the following text: (requires login)
For both SDL Tridion World and the SDL WCMS Documentation portal you could use the following snippet in your answer (works in comments too):
Additionally I've seen we (well Alvin mostly ;o) also provide links to the forum (https://forum.sdltridion.com/) which currently also still requires a login. For that one I suggest we use the following link (the docportal is really only meant for Live Content):