Add a generic interface function `interpolation` that hooks into the
stage after macro expansion so that interpolated values in docstrings
can implement their own behaviour rather than being directly
interpolated into the docstring. Pass the documented expression through
to `interpolation` such that abbreviation implementations can make use
of this expression for their own needs.

Co-authored-by: Morten Piibeleht <morten.piibeleht@gmail.com>