Only match first identifier in doc tag

Well. That took a lot longer to find time for than anticipated.

This follows up on #19 by matching only the first identifier, and only if it occurs as the first or second argument and is preceded by at least one whitespace character.

I could not get the `\@<=` construct to work in the syntax file even though it worked in manual searches. I would have preferred to use that for the preceding `\s` in lines 585 and 586.