Use Arg::value_name() as header for positionals in clap_mangen

[ducaale]

This was split out from #3580

[ducaale]

There are two other things I would like to address for positional arguments but I am not sure if I should open a separate PR or not:

• Don't show hidden flags in the SYNOPSIS sections.
• Include triple dots if positional arg can be more than one item.

Edit: I would also need to fix any failing tests.

[epage]

Don't show hidden flags in the SYNOPSIS sections.
Include triple dots if positional arg can be more than one item.

As these are all independent changes, I'd prefer separate PRs

I would also need to fix any failing tests.

Let me know if you have any problems with the snapshot testing (or any feedback on the process!)

[epage]

Thanks!

[ducaale]

What would be the best way to preview .roff files? I am trying to compare the differences in snapshots before updating them with SNAPSHOTS=overwrite

$ man tests/snapshots/aliases.bash.roff

my-app(1)                                      General Commands Manual                                      my-app(1)

NAME
       my/-app /- testing bash completions

SYNOPSIS
       /fBmy/-app/fR      [/fB/-h/fR|/fB/-/-help/fR]     [/fB/-V/fR|/fB/-/-version/fR]     [/fB/-f/fR|/fB/-/-flag/fR]
       [/fB/-o/fR|/fB/-/-option/fR] [/fBpositional/fR]

DESCRIPTION
       testing bash completions

OPTIONS
       /fB/-h/fR, /fB/-/-help/fR
              Print help information

       /fB/-V/fR, /fB/-/-version/fR
              Print version information

       /fB/-f/fR, /fB/-/-flag/fR
              cmd flag

       /fB/-o/fR, /fB/-/-option/fR
              cmd option

       [/fBpositional/fR]

[epage]

Thanks!