Documentation: Fix irq-domain.rst build warning

Correctly escape the * not to be used as emphasis. Also take this
opportunity to clarify the fate of the rest of the legacy APIs.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20210903085343.923036-1-maz@kernel.org