feat(docs): format parameters as a list #20485

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.

scripts/gen_vimdoc.py

@@ -673,7 +673,7 @@ def fmt_node_as_vimhelp(parent, width=text_width - indentation, indent='',
         max_name_len = max_name(m.keys()) + 4
         out = ''
         for name, desc in m.items():
-            name = '    {}'.format('{{{}}}'.format(name).ljust(max_name_len))
+            name = '  • {}'.format('{{{}}}'.format(name).ljust(max_name_len))
             out += '{}{}\n'.format(name, desc)
         return out.rstrip()