docs: clarify cmd_psuh signature and explain UNUSED macro

The sample program, as written, would no longer build for at least two
reasons:

 - Since this document was first written, the convention to call a
   subcommand implementation has changed, and cmd_psuh() now needs
   to accept the fourth parameter, repository.

 - These days, compiler warning options for developers include one
   that detects and complains about unused parameters, so ones that
   are deliberately unused have to be marked as such.

Update the old-style examples to adjust to the current practices,
with explanations as needed.

Signed-off-by: K Jayatheerth <jayatheerthkulkarni2005@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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