doc: Update docstrings for built-in functions to not use "Loadable Function" as type. * data.cc, debug.cc, help.cc, ov-oncleanup.cc, sparse.cc: Update docstrings for built-in functions to use "Built-in Function" as type.

function x = varg_nest (varargin)
  x = abs (f (-5)) + g;

  function x = f (varargin)
    x = abs (varargin{1});
  endfunction

  function x = g
    x = abs (varargin{1});
  endfunction
endfunction