First hit for googling "R api" is "Writing R Extensions". Looks pretty easy googling for R to me.
Actually that would be a neat search engine idea: Targeted at programmers, you can specify the language the library/question/tutorial/etc. should be written in and it searches only websites related to this.
It's not a search problem, as much as it's an indexing one, no?
True. Should be basicly doable like a filter on google