tools: flatten apidoc headers

ensure optional parameters are not treated as markedown links by
replacing the children of headers nodes with a single text node
containing the raw markup;

Fixes issue identified in
https://github.com/nodejs/node/pull/21490#issuecomment-406859983

PR-URL: https://github.com/nodejs/node/pull/21936


Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Minwoo Jung <minwoo@nodesource.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>