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

Documentation/core-api/irq/irq-domain.rst

@@ -175,9 +175,10 @@ for IRQ numbers that are passed to struct device registrations.  In that
 case the Linux IRQ numbers cannot be dynamically assigned and the legacy
 mapping should be used.
 
-As the name implies, the *_legacy() functions are deprecated and only
+As the name implies, the \*_legacy() functions are deprecated and only
 exist to ease the support of ancient platforms. No new users should be
-added.
+added. Same goes for the \*_simple() functions when their use results
+in the legacy behaviour.
 
 The legacy map assumes a contiguous range of IRQ numbers has already
 been allocated for the controller and that the IRQ number can be