Format parameters in docs using <dl>

This patch formats the parameter section of the Doxygen documentation produced by API using <dl>. This makes it easier to read, as the parameter name stands out from the descriptions, instead of running into each other. While I was at it, I factored the parameter parsing code out of api_parse_php_file.

My one main question is whether the pattern to extract the parameter name is correct. I used \w, which seems to match the valid PHP name characters including things like è, but I'm not 100% positive about it.