octave: generate the documentation

I find octave quite unusable, without its help pages. Recently it started to
require gnuplot to generate them (even the text-only).

I also enabled parallel building. A comment from 2012 said about spurious
failures if it were enabled, but I don't know if these happen again. Let's
enable it to see.

(cherry picked from commit a6f1024b1f2235825d7b441125db2e0d42febdd1)

Conflicts:
	pkgs/top-level/all-packages.nix