Main conformance: clarify floating point accuracy

Clarify how to compare with an error range of a given
number of ulp in this specification. Denormals may be
flushed to zero.

Signed-off-by: Dominic Symes <dominic.symes@arm.com>
Change-Id: I1158e777030c4770d74f1acff84ab98e4a3420ac