> Note: I'm not yet savvy enough with the source to sort-out why
> `self.locals.get` is returning `None` instead of an empty string.
>
> But this seems to work well enough for me 🤷

The result of `function_arg` Regular Expression now uses labels,
however, because we can have a list of comma separated arguments to
parse this requires a _little_ bit of list comprehension.

> Note: in the future it'd be nice to have `@default` automatically get
> assigned when `args_groupdicts` has both `key` and `value` are not
> `None`, but at this point I'm happy enough to have Vim 8 syntax
> supported when building help documentation.

These changes _should_ prevent breaking _long words_, such as URLs or
function names...  But detecting when within a code-block/example is,
for now, an exercise I've been advised to be a bit apprehensive about
tackling with a larger re-write.