IVGCVSW-5335 Documentation for fast_math
IVGCVSW-5335 Documentation for fast_math

 * Changed documentation for fast_math to add warning about possibly reduction in precision.

Signed-off-by: Mike Kelly <e10d499d2073d26b149e114dc8420d8f96d919f9@arm.com>
Change-Id: If954471efc6aef702e8490585571815d9e19b3fc

ARM-software/armnn,ARM-software/armnn,ARM-software/armnn