There are now two versions of the Arnold User Guides. For example:

Arnold for Maya 4 User Guide

Arnold for Maya 5 User Guide

If you are not sure which version you are on, have a look at the top and it should say which version, for example: arnold for maya user guide 5

Also, the url for Arnold 5 has an A5 prefix, like so:

https://support.solidangle.com/display/A5AFMUG/Arnold+for+Maya+User+Guide