documentation: Update Gosub, Goto, and add new documentationtype.

documentation: Update Gosub, Goto, and add new documentationtype.

Gosub and Goto were not displaying their syntax correctly on the docs
site. This change adds a new way to specify an optional context, an
optional extension, and a required priority that the xml stylesheet can
parse without having to know which optional parameters come in which
order. In Asterisk, it looks like this:

  parameter name="context" documentationtype="dialplan_context"
  parameter name="extension" documentationtype="dialplan_extension"
  parameter name="priority" documentationtype="dialplan_priority" required="true"

The stylesheet will ignore the context and extension parameters, but for
priority, it will automatically inject the following:

  [[context,]extension,]priority

This is the correct oder for applications such as Gosub and Goto.