gh-104683: Argument Clinic: Refactor and simplify 'add docstring' states (#107550)

Introduce docstring_append() helper, and use it for both parameter and
function docstrings. Remove docstring fixup from
do_post_block_processing_cleanup(); instead, make sure no fixup is needed.

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>