scripts/kernel-doc: Replacing highlights hash by an array

The "highlight" code is very sensible to the order of the hash keys,
but the order of the keys cannot be predicted. It generates
faulty DocBook entries like:
	- @<function>device_for_each_child</function>

Sorting the result is not enough some times (as it's deterministic but
we can't control it).
We should use an array for that job, so we can guarantee that the order
of the regex execution on dohighlight is correct.

[jc: I think this is kind of papering around the real problem, that people
 are saying @function() when "function" is not a parameter.  But this makes
 things better than they were before, so...]

Signed-off-by: Danilo Cesar Lemes de Paula <danilo.cesar@collabora.co.uk>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>