CodingGuidelines: also mention MAYBE_UNUSED

CodingGuidelines: also mention MAYBE_UNUSED

A function that uses a parameter in one build may lose all uses of
the parameter in another build, depending on the configuration.  A
workaround for such a case, MAYBE_UNUSED, should also be mentioned
when we recommend the use of UNUSED to our developers.

Keep the addition to the guideline short and document the criteria
to choose between UNUSED and MAYBE_UNUSED near their definition.

Signed-off-by: Junio C Hamano <gitster@pobox.com>