Documentation is one of the many aspects of software products where one can never be perfect. You can always document more, and finally, even if you do, there will be too much information - hard to find and organize and most importantly, hard to keep up-to-date.
I'm not going to say that telerik's docs are perfect, but fom my experience with 3rd party vendors, they are (arguably of course) one of the best. Of course, they can always improve.
Weak spots in the documentation are the client-side and server-side APIs. For example, when I look up for a specific property in the API help, I expect short explanation and hopefully a code snippet, or at least a cross-link to a section in the help where I can find related (to this property or method) information. 90% of the time this is not the case and I end up searching forums, making up clever Google search phrases and using hardcore debugging tricks to find out what is going on.
For some products this is especially visible. rad Window for example uses a somewhat different client-side approach (compared to other telerik products) and the docs are needed a lot. However, in the current state of the docs you cannot even see if a member of a client-side object is a property or a method, and code examples are virtually non-existant.
But then again, this is one of the few examples where help is lacking. Generally, help & examples are quite, quite good.