this is among the more important features we want to implement, but it will surely not be available in the upcoming Service Pack.
The reason for indexers section in the web.config is that like anything else in Sitefinity, search indexes can be developed and implemented by clients (e.g. you develop some module with complex data and you want to handle the way it'll be searched on your own, you'll be able to implement your own searching logic), so that's how you would register it. Unfortunately the exact process or the tutorials for this are not yet available and it's not exactly a trivial thing to do.
To conclude: once implemented, the PDF and DOC search will search the contents of a file. It will not be necessary for this file to be in the Images & Documents library. And finally, we don't have a fixed date of this release, but we are working on it.
the Telerik team