Dummy argument links

Hello, see the following example case:

```f90
module mod
integer, dimension(2) :: arg
!! This has an anchor [[mod(module):arg(variable)]]
contains
subroutine sub(arg)
integer, dimension(:), intent(in) :: arg
!! This has an anchor, but how to link to this? [[sub(subroutine):arg(variable)]] does not work.
end subroutine sub
end module mod
```

I'm wondering how to link to the `sub`'s dummy argument `arg`. An anchor is being created anyway, see:
```html

integer,

intent(in),

dimension(:)

::
arg

This has an anchor, but how to link to this? sub does not work.

```

Is it possible to link to the dummy argument somehow? I'm a developer in a Fortran framework, and it would be helpful sometimes to have an indicator where certain arguments come from, e.g. the output of a normalization routine or some other step in a pipeline.

I can still use a macro that expands to sth. like `'arg' from [[mod(module):sub(subroutine)]]`. I was just wondering if there might be a Ford-native way

Start with Ford's handling of the `[[sub(subroutine):arg(variable)]]` reference syntax and compare it with the generated `variable-arg~2` anchor in the HTML output. Done means a documented or working Ford-native link to the subroutine's dummy argument, while preserving the existing module-variable link behavior.

Written by the indexing model from the issue text.