Document pzstd -p

[bmwiedemann]

Document pzstd -p in man-page
so that users get at least the same information as from pzstd --help

[Cyan4973]

pzstd is not part of the standard zstd package. It's an additional tool.
Maybe it could have its own man page, but I don't think it's a good idea to mix it within the zstd.1 man page.
cc @terrelln .

[Cyan4973]

Closing.
If we get to create a man page for pzstd, it will be a separate one from zstd.