main addition clean up

added main to flow content element list, removed example code heading ,
removed palpable content as main category (harmonize with other
elements) fixied some copy/paste errors, moved user agent note, trried
to fix linking of main in code example
added main to special parsing rules list

refer to
http://www.w3.org/html/wg/drafts/html/master/single-page.html#the-stack-of-open-elements
+ add back palapable as category for main, to align with other grouping
content defs.
fix conflict with addition of main