#1 Give examples of batch capture using capture_demo

[Anthony Bailey]

The capture_demo command exits on completing a capture.
The idea is that the user can perform batch capture of
(multiple versions of) multiple demos from their shell,
which will be more versatile than using the Quake console.

However, this way of working is not going to be obvious
to all users. So I should explain the intent and
document some example uses.