doc/: Reorder the arguments to `\descref'.
We now have
\descref*{CAT}{NAME}[TEXT]
rather than
\descref*{NAME}[TEXT]{CAT}
which flows much better. There's a complication, that `\@ifnextchar'
gobbles space while looking for its character: implement a custom
version which doesn't do that.
The call sites were mostly fixed by [long line]
$ sed -i '/\\descref\(\*\)\?\({[^}]\+}\(\[[^]]\+]\)\?\)\({[^}]\+}\)/ s//\\descref\1\4\2/g' doc/*.tex
but some calls run over two lines and had to be found and fixed by hand.