perl: Enhance documentation for example plugin.

This fixes commit 6edf6b4e261011342c40e871231d8430db31f490.
perl: Move nbdkit-perl-plugin man page to section 3.

Since it's a programming man page.

This fixes commit 6edf6b4e261011342c40e871231d8430db31f490.
perl: Enhance nbdkit-perl-plugin man page.
docs: Link to nbdkit-perl-plugin(3) in the nbdkit-plugin(3) man page.
docs: Fix man page name in various plugin man pages.