CodingGuidelines: mention -Wunused-parameter and UNUSED

Now that -Wunused-parameter is on by default for DEVELOPER=1 builds,
people may trigger it, blocking their build. When it's a mistake for the
parameter to exist, the path forward is obvious: remove it. But
sometimes you need to suppress the warning, and the "UNUSED" mechanism
for that is specific to our project, so people may not know about it.

Let's put some advice in CodingGuidelines, including an example warning
message. That should help people who grep for the warning text after
seeing it from the compiler.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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