Handle negative prefix arguments to mark-paragraph correctly

Handle negative prefix arguments to mark-paragraph correctly

* textmodes/paragraph.el (mark-paragraph): Handle negative
arguments correctly (bug#18847).  This makes `M- M-h M-h' do the
correct thing with expanding the region (like other marking
commands) backwards.  Also fix problem at the end of the buffer,
where the numbers of paragraphs left in the buffer is less than
ARG, then paragraphs would also be marked *before* the current
paragraph.  Also clarify the doc string.