plpgsql-trigger.html: Format TG_ variables as table (patch)

Hi,

I found the list of TG_ variables on
https://www.postgresql.org/docs/current/plpgsql-trigger.html#PLPGSQL-DML-TRIGGER
hard to read for several reasons: too much whitespace, all the lines
start with "Data type", and even after that, the actual content is
hiding behind some extra "variable that..." boilerplate.

The attached patch formats the list as a table, and removes some of
the clutter from the text.

I reused the catalog_table_entry table machinery, that is probably not
quite the correct thing, but I didn't find a better variant, and the
result looks ok.

Thanks to ilmari for the idea and some initial reviews.

Christoph