doc/: Reorder the arguments to `\descref'.

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.