Unfortunately we must admit your observations are correct -- due to a glitch within the help scripts implementation the documentation for some Silverlight controls (e.g. Chart, Gauge, GridView) is not merged properly and does not register under the "RadControls for Silverlight" node.
While we are already working on a solution for this problem and this is our top priority for the first service pack release of the controls, for the time being you can download the "CHM Help" zip file (~30 MB) from the RadControls for Silverlight section in your Client.Net account, unzip it, and start the "RadControlsForSilverlight.chm" file in the main folder -- it contains the proper and latest documentation for all controls.
Sorry for the temporary inconvenience.
the Telerik team