Add a note to toplevel READ with RECURSIVE-P set

[phoe]

07:59 < beach> "If recursive-p is true, the call to read is expected to be 
               made from within some function that itself has been called 
               from read or from a similar input function, rather than from 
               the top level. "
08:00 < phoe> Yep, that is what I quoted before.
08:00 < beach> Oh, sorry.
08:00 < beach> Missed it.
08:00 < phoe> Therefore it might be UB if there is no non-recursive READ on 
              the stack but you call a recursive READ.
08:00 < beach> So I take that to mean "the consequences are undefined if READ 
               is called with RECURSIVE-P being true at the top level".