[DEPRECATE] pretext/pretext script, new "doc" component

[Rob Beezer]

Short version: "doc" is now a synonym for the "all" component when using the
pretext/pretext script. (This is NOT the CLI you have installed as a Python
pip package).

The Python pretext/pretext script is useful for developers, and those who like
a Swiss Army Knife approach to their command-line tools. Originally, it was
meant to rip out "components" for external processing (think TikZ images
processed by LaTeX). As we needed more Python (since XSL was not good at
certain things), it has come to be used to build entire documents. So "-c doc"
is now enabled as a switch, in lieu of "-c all". The latter could have been
confusing since it doesn't really create components, and there is already an
"all" option for *formats*.

You won't get warnings, and the old way still "works", but the (minimal) help
text now lists "all" as deprecated.

(Oscar, et alia: this should have no effect on *consumers* of the underlying
module/library. This just affects the bare-bone user interface to those routines.)