The fact the issue arose in fltk.discussion is proof that the reference to the "current path" present in the function's one-line description is not enough. I would suggest to slightly rephrase the first two sentences of the added text as follows to connect it to the "current path":

    Unlike the integer-coordinate \c fl_arc(x,y,w,h,a1,a2) overload, this function requires a current path, that is, 

    it must be bracketed by matching fl_begin_*() and fl_end_*() calls.