elem: Fix lint and generate issues (hexops#205)

* Fix heading title detection

* Fix Heading{1-6} function comment sentences

* Remove debugging line

* Filter out invalid (redundant input) elements

* Handle additional description styles, special cases

Uses a couple regular expressions, and a couple special cases for
<audio> and <h{1-6}> now.

* Handle uppercase Element in regexpr as well

* Fix using only one element of split desc

* Cleanup some description suffix irregularities

* Add back <tagname> instances to description

The first one is replaced with funName, the rest were not added back and
were blank. This is a bug fix.

* Handle descriptions with "(" + "(E|e)lement"