Updated filetype detection, syntax comments

== asciidoc_filetype.vim
  * (a) changed algorithim to detect asciidoc filetypes in ambiguous file
  * extensions (e.g. *.txt).  Used updated asciidoctor specifications to
  * detect one- and two-line titles at start of file.
== asciidoc.vim
  * (a) changed erroneous comments describing syntax regexp