Problem:
The {foo} parameters listed in `:help api` and similar generated docs,
are intended to be a "list" but they aren't prefixed with a list symbol.
This prevents parsers from understanding the list, which forces
generators like `gen_help_html.lua` to use hard-wrapped/preformatted
layout instead of a soft-wrapped "flow" layout.

Solution:
Modify gen_vimdoc.py to prefix {foo} parameters with a "•" symbol.

(cherry picked from commit 24e5b5c)