Commit d0e03f09 introduced GTYPE_TO_POINTER/GPOINTER_TO_TYPE, on which
work started before the move of conversion macros to Markdown. It did
get merged after that move, but the macros never made it into the docs.
Let's do so now.
There is no need to use double backticks. Also, insert extra newlines
between header and item, which paragraphs the item [1, 2], increading
readability by giving a little more "breathing" room between items. This
is also done in `macros.md`. Also, indentation of the definition block
should technically be 4 if extra paragraphs are needed, but it's better
to use 4 indentation always to prevent future problems after edits.
[1] https://python-markdown.github.io/extensions/definition_lists/
[2] https://michelf.ca/projects/php-markdown/extra/#def-list
