SDEI: Add prioritisation clarification

To make exception handling amongst Secure components, require that SDEI
exception priorities must be assigned the lowest among Secure
priorities. Clarify documentation to this effect.

Change-Id: I92524b7b7e9b3fa06a10c86372bc3c4dd18c00ad
Signed-off-by: Jeenu Viswambharan <jeenu.viswambharan@arm.com>

docs/porting-guide.rst

@@ -2041,9 +2041,9 @@ This macro must be defined to the EL3 exception priority level associated with
 Critical SDEI events on the platform. This must have a lower value (therefore of
 higher priority) than ``PLAT_SDEI_NORMAL_PRI``.
 
-It's recommended that SDEI exception priorities in general are assigned the
-lowest among Secure priorities. Among the SDEI exceptions, Critical SDEI
-priority must be higher than Normal SDEI priority.
+**Note**: SDEI exception priorities must be the lowest among Secure priorities.
+Among the SDEI exceptions, Critical SDEI priority must be higher than Normal
+SDEI priority.
 
 Functions
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