arm64/signal: Document our convention for choosing magic numbers

Szabolcs Nagy has pointed out that most of our signal frame magic numbers
are chosen to be meaningful ASCII when dumped to aid manual parsing. This
seems sensible since it might help someone parsing things out, let's
document it so people implementing new signal contexts are aware of it and
are more likely to follow it.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20221031192450.826159-1-broonie@kernel.org


[will: Fixed typo and tweaked wording]
Signed-off-by: Will Deacon <will@kernel.org>