This adds feature parity for mangen with the standard help output. Users
will now see the list of possible values for value arguments.

One change that was made to make this possible was adding the method
`get_possible_values` to the public API for an arg. I tried to think of
a way to get around this, but because this is the interface that the
help generation uses, and it is part of the crate public interface
I thing adding it as a part of the public API might be for the best.

cherry-pick to backport this fix to v3

fixes: clap-rs#3861

This makes sure we take into account the setting that possible args
is hidden

backporting to v3-master

Noticed that possible values would not show up for Positional arguments
as well. Decided to add the changes for those as well.