arch: aarch32: document exception priority scheme for 32-bit ARM

This commit adds some documentation for the exception
priority scheme for 32-bit ARM architecture variants.
In addition we document that SVCall priority level for
ARMv6-M is implicitly set to highest (by leaving it as
default).

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>