System Information
Windos 10 + Nvidia GeForce 1060 6Gb

Blender Version
2.79b

Short description of error
Each button for documentation of each add-on within the User Prefs sends to an empty webpage, so no documentation can be easily found.

Exact steps for others to reproduce the error
Just open the User Settings + Add-ons tab, expend each add-on (active or not active, the same) and try clic onto every [Documentation] button.

Suggestion : maybe an automatic redirection webpage on the web side re-sending on the good link will be easier than to fix the links within Blender...