Only generated <param> elements if the parameter is in the overloaded C#
function.
Copy implementation from javadoc.cxx.
Also fixes generating an incorrect '...' parameter for vararg functions.
Which fixes failing CI where Microsoft csc.exe (from VS2019 at least)
converts:
/// <param name="..."> extra args</param>
into xml with '......' as the parameter:
<param name="......"> extra args</param>
failing testcase doxygen_basic_translate which checks parameter names.