Clarify PSA_PAKE_STEP_ZK_PROOF format
Clarify PSA_PAKE_STEP_ZK_PROOF format

Removing reference to RFC 7748 as it is more confusing than helpful. (It
decodes the scalars after masking which is not part of the encoding we
want to specify. Also, it has the explanation what it means by little
endian in a preceding section that is not trivial to find.)

We also explicitly specify constraint on leading zeroes.

Signed-off-by: Janos Follath <b175c17abe0474719672be1367bb75ee28bd5c71@arm.com>

NXPmicro/mbedtls,Mbed-TLS/mbedtls,ARMmbed/mbedtls,Mbed-TLS/mbedtls,ARMmbed/mbedtls,NXPmicro/mbedtls,Mbed-TLS/mbedtls,NXPmicro/mbedtls,Mbed-TLS/mbedtls,ARMmbed/mbedtls,NXPmicro/mbedtls,ARMmbed/mbedtls