[R6RS] continuable exceptions

[William D Clinger]

In addition to several minor corrections to basic.tex,
I made these significant changes, which I believe to be
corrections:

I reworded the language on exceptions not being
continuable to reflect my understanding of the
matter, which is that implementations are allowed
to make exceptions continuable even when that is
not guaranteed by the R6RS.

I also deleted the subsection on "Multiple return
values" from terminology.tex, because it was exactly
the same as the subsection of the same name in
basic.tex.

Will