NOTPARALLEL rule for documentation directory

[Rik]

8/28/12

John,

I noticed in the discussion of octave.html no longer building that the
.NOTPARALLEL directive was commented out in the same changeset
(http://hg.savannah.gnu.org/hgweb/octave/rev/017f0b2e6933).  Has something
changed in Automake to make this directory parallel-safe?  Otherwise, I
think it is still needed.  As the comment says, it isn't our Makefile rules
that are failing, but the auto-generated ones which we don't have control of.

--Rik