Built-in variables documentation

[Søren Hauberg]

Hi,
  The chapter in the manual about variables has two sections (7.4 and
7.5) on built-in variables. These were changed into built-in functions
quite some time ago, so this part of the manual should really change.
  I see two possible changes:

1) Create a new (fairly short) chapter on customising Octave. Here we
could describe the built-in functions.

2) Move the descriptions of the built-in functions to different
chapters, i.e. move the 'page_screen_output' description to Chapter 14
[Input and Output].

Personally, I prefer the second solution. Any alternative solutions or
opinions?

Also, we have "Variable Index" at the end of the manual. Perhaps that
should just be removed now, that almost all variables have been removed?

Søren